Well Ladies and Gents, I need your help.
(I have some questions at the bottom.)

Here's the TL:DR-

whilst playing various games i keep experiencing powerloss style reboots on my pc,
no blue screen, no dumps just powerloss and then it reboots.

The details:

As far as i can tell through live monitoring and logs all thermals are okay. The CPU stays cool as a cucumber as does the GPU.
This actually started little by little a year ago but due to a shit load of fun stuff this year it kept being put on the backburner.
I've had this rig set up since May 2015 no changes apart from the update to win 10 later that year.

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Processor Information:
CPU Brand: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz

Motherboard:
Asus Maximus VII Ranger

BIOS:
2902

Operating System Version:
Windows 10 (64 bit)

Video Card:
Driver: NVIDIA GeForce GTX 970
Monitor Refresh Rate: 75 Hz
Number of Monitors: 1
Number of Logical Video Cards: 1
Primary Display Resolution: 2560 x 1440

Sound card:
Audio device: Speakers (Logitech Speaker)

Memory:
RAM: 16326 Mb

PSU:
Corsair CX750M
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The only old piece of equipment is the PSU from December 2013.
As far as i can tell i have more than enough headroom on the 750w PSU, i have a smart meter monitoring my home and i can see the live power consumption at all times on the panel next to my PC, and when i'm gaming at night, when i know everything is switched off, the total wattage
never goes above about 550w and that actually includes a few dimmed lights and a lava lamp too.

The GPU had been sat at a comfortable and well tested overclock for nearly three years until about a year ago when this first happened.
It was maybe an odd choice but for some reason because i was playing a fairly gpu heavy game for a 970 my gut instinct was to pull back my overclock.
I think it makes sense, overclocked pc with power headroom suddenly loses power, my first thought was could it be an overclock affected and made unstable by a driver update?
This seemed to hold it off in that game (i believe one of the new Tomb Raiders on Ultra) and it didn't happen for a while.
Then over the year it kept happening, often at the same points in games so i could replicate it. It was usually as something was loading up ready to happen out of sight of the character type thing, you know like your character is walking down a corridor and it triggers the game to load more of the game as you walk.
For each game that it happened on I found pulling back the overclock seemed to help until about 6 months ago when i had finally reduced the overclock so much that i had to just set the GPU all the way back to default to stop it happening.

It hadn't happened again until this maybe two weeks ago when i started playing Soma, and even with no overclocking it keeps happening.

I've got to be honest, whether it's the GPU or the PSU i was looking at buying an RTX 2060/70 anyway so i guess if it kept happening after I bought a new GPU i could just replace the psu too.

The question I suppose is, considering i don't tend to have problems at any other times, is it likely to be the GPU or PSU?
Has anyone had anything similar happen to them?
Is it likely to be something totally different that i have no idea about like dodgy RAM (literally just throwing it out there) or something?

I think this is the heart of your problem right there. Possibly the capacitors in your PSU are at end-of-life. Observe:

"A bad capacitor can cause a variety of mischief in computer circuits. Your computer may spontaneously restart, it may fail to start occasionally or completely. It may fail to finish the power-on self test (POST), which it performs before starting the operating system. When you turn the computer on, its fans may start, but otherwise it does nothing. While computer software problems tend to be consistent, hardware problems like this cause erratic, unpredictable behavior."

https://www.techwalla.com/articles/symptoms-of-bad-capacitors

Temporary solution:

Reduce the power consumption of your computer. Putting the computer into power saving mode is one option. Underclocking the CPU and GPU, if you are able, is another. (At the very least, turn off your A third option, of course, is to switch to using the integrated GPU instead of your discrete graphics card, as discrete GPUs, especially on heavy load, use more power. Of course, doing this will reduce gaming performance, so you may need to turn down settings (turning down the settings might make this less likely to happen even without underclocking, provided the game is well behaved and doesn't waste CPU/GPU cycles).

The long term solution, of course, is to replace the faulty part. (TARFU seems to suggest that it's the PSU), or just get a new computer.

Yep, more than likely the psu. You might want to list your full system specs including make, model wattage of your psu for proper context.


Edit:

Oops, I missed your specs in the op. Post the make, model and wattage of your psu.

I had a similar issue a few years back with one of my builds. At the time I had no spare parts to test out components. What I ended up doing was underclocking the CPU, which increased stability on the system to the point where I had no random reboots. I eventually ended up getting a new system and was able to test the components apart from each other on another motherboard. It ended up being the motherboard that was causing my issues.

Seems odd the PSU looks like it should be able to easily support the Nvidia 970

What are the actual temps and how are you measuring them? Have you used a few independent tools to compare if something is 'amiss' between reporting tools

Testing temps should be simple to do by islating components to stress test

Use Prime95 and stress test teh CPU. Monitor temps and see if that 'blips' the computer

Then try the same on your GPU. EVGA has a few simple tools that stress the GPU.

I do semi-concur with the other posters that it kinda sounds like your PSU, but you should try to maybe confirm temperatures via independent tools to see ifyou're getting some bad data

Certainly your PSU. PSU can be very weird. My old one seemed to be fine until I started playing Doom '16 and then all sorts of weird shit happened until it finally started doing what you're saying your pc is doing. You can get a pretty good one for cheap, so certainly start with that. If that doesn't fix it then you know it's the cpu. CPUs can take abuse. I'd wager on your PSU.

Sounds like a PSU problem. I had a similar issue a year ago, solved it by buying a new PSU and replacing the thermal paste on the CPU. At first I tried monitoring temps, cleaning out dust, and cranking down game settings. That worked for a little while but eventually I spent some money and have had no major issues since.

Test your PC using https://www.userbenchmark.com/ and then post the results here.

Did you disable that silly Win option to restart automatically without showing the blue screen? (for Win7 it's inside System Properties - Startup & Recovery)
Also, check you temperatures while gaming (for example set alarms with MSI afterburner).

It could be motherboard or PSU related. I'd definitely try and test it for a week with another PSU first though. Let me explain why:-

Bronze PSU's tend to come with 85c rated capacitors whilst Gold / Platinum often come with 105c rated ones. The life of capacitors also tends to halve per 10c temperature increase vs its rating (it doesn't matter whether it actually reaches 85-105c inside the PSU, what it means is that all else being equal, eg both have a 55c ambient temp, 105c capacitors Gold PSU's have a tendency to last 3-4x longer in practise vs 85c Bronze / White grade PSU's due to degrading less over time). In reality, temps are lower in the more expensive PSU's due to higher efficiency (which further improves lifespan). Eg if both have components that are drawing say 175w from the PSU:-

Bronze : 175w DC @ 82% efficiency = 213w AC or 38w waste heat.
Gold : 175w DC @ 92% efficiency = 190w AC or 15w waste heat.

The extra +23w on the Bronze is additional waste heat generated inside the PSU vs Gold due to the lower efficiency during AC-DC conversion (on top of already weaker capacitors). PSU's also don't need to explode / hard fail to go bad. I used to own a Bronze and after 4 years it randomly rebooted. Swapped to a Gold and I've never had a single OS crash / unwanted reboot since. Now it's possible that it is the motherboard rather than the PSU, but that's only something you can test for if you have a second PSU / mboard. Personally though, I'd never go back to using just Bronze rated PSU's, and the typical 3-5 year Bronze vs 7-10 year Gold / Platinum warranties speak for itself.

It is probably the PSU, I had very similar problem, but my computer would reboot even when just browsing the web, and it was almost impossible to replicate. Then it suddenly worked fine for two months, before it started happening again. My PSU was just 6 months old. If it was the GPU, you would probably lose image first, and after few seconds the PC would reboot. It is very hard to diagnose this stuff without proper equipment or spare parts.

PSU or mobo. Nothing else would have these symptoms. Try a different power supply and hope that is it. I hate having to change mobos....

Aside from PSU and motherboard, faulty RAM can also lead to seemingly random restarts. Since this is the easiest component to test, I recommend to start there and then move on to the other components if the RAM is okay.