Well crap. I just had a random reboot. The system was basically idle -- I was just editing a text file in vi, with basically nothing running in the background. All of a sudden screen goes black and system reboots. No errors, nothing to go on in the logs :(

Sounds like you might have a bad motherboard... apparently the motherboard makers were a little upset with AMD, they had to rush their products to market.

https://mobile.twitter.com/CPCHardware/status/843099198799187968

Looking even more promising.

"There are many Ryzen 16C / 32T focused HEDTs planned on X399 in 4/6 months. Clocks ~ 2.4 / 2.8G. 2 dies MCM. 4 chan DDR4. Socket LGA SP3r2. ~ 150W."

That's the exact RAM/ MB combo I've been looking at so it would be disappointing if they don't play nice together. I'm waiting until the 500 series releases to buy though since I need a new graphics card as well, hopefully by then some of the problems will be ironed out. ASRock seems to have the best reputation for their Ryzen MBs (ironically given your troubles, Gigabyte second) but there doesn't seem to be a single AM4 ASRock MB in the entirety of New Zealand yet and from what I've seen even in the US they get snapped up as soon as they become available.

There seems to be different versions around of this Corsair RAM, with Samsung or with SK Hynix chips on it. Samsungs may work better than SK Hynix. Dunno how to determine the version of my RAM though. Analysis tools cannot read it from SPD or something like that. No indication on the package the RAM came with either.
Maybe the X370 chipset is working with better support than the B350.

Just guessing and waiting for BIOS updates I would say. At the moment I don't feel the need for faster RAM anyway because the system is faster than anything I had before. I am happy for now, but you know how it is if you know that there is still a potential for even better performance. :-D

It is far too early to jump into such a conclusion. Every part of the build is new, any one of them could be malfunctioning. Or it could just be a matter of tweaking something (dram timings again?) in bios, knowing the ryzen's memory controller is a little picky. Heck it could be a software problem. In fact I just swapped to another OS and the machine's been running for 15 hours without a reboot now. Maybe it helped. Time will tell.

I have a daily reboot with a Bluescreen for an unknown device. Driver Power State Failure, since I run the computer on Max Power in the System Energy Settings, rather than "balanced". It always just one time, so I guess some device is not correctly shut down on the previous day and causes a wake-up error. I have to pin down what it is, but I guess it is one of the network devices.

I have now finally got my RAM to run at 2400 MHz with rather aggressive timings (14-14-14-14-28). The trick was not to use the XMS profile of SPD, but that means the RAM is only running at 1,2 V rather then the specified 1,35 V. Well for now it seems stable (on Desktop, have to run some games now), but fact is the timings seem to be tricky beast as well. One less and Windows refuses to start, two less and the BIOS resets.

Well it is developing, what was to be expected as an early adopter. ;-)

If anyone's still hoping there's something between the RX480s & the Vega cards, things look more promising.

https://videocardz.com/67503/amd-polaris-12-spotted-with-640-stream-processors

Jayztwocents is playing & streaming using an Ryzen 1800x.
Should be interesting to see things as they happen.
Already had problems with his software not capturing DOOM hehe.

https://www.youtube.com/watch?v=VL2TAJvfiQU

Links please, there is no ECC menu in UEFI, and ECC without scrubbing is a non-ECC mode.

Well, I thought my system was stable now. After 10+ hours with no error in memtest86+, I installed and played through shadow warrior 2013, the entire game, running the OS for a few days with no issues (after switching to Fedora -- it turns out the random reboots happen on Arch only). Unfortunately this morning I woke up to a blank screen with no signal -- frozen system, with yellow dram fault led on mobo lit. After a pressing reset the system boots and everything is running normally again but I don't see why the same issue wouldn't happen again in a day or seven. Annoying, and hard to diagnose. Clearly still not stable :(

I wish I could check CPU temps and RAM timings under Linux somehow.. *sigh*

Not sure, be check if Logitech Gaming Software has Linux port & if it does, see if can use ARX.
If not, Roccat Power Grid I think has a Linux port. It can also tell you temperatures etc.

Need an android mobile connected via wi-fi (for Roccat) or any connection for ARX.

Doesn't Linux have HWINFO? If it does, that will show you anything you'd possibly need.

AMD did a splendid job with RYZEN.
Literally BEASTS for a great price!

The leap they managed on IPC is crazy. Will buy a RYZEN 2 or 3 when they come out, as there is no huge gain for me to upgrade now.

Also waiting for the VEGA GPUS. The day my case will have only AMD C/GPUs in it, is getting closer.

the linux kernel doesn't yet have sensor support for the ryzen cpu.