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I have a gtx 750 ti 4 Gb ram
for some reason, the FPS is very unstable in the game. most of is 60 but some parts drops to 45.

I tried the lowest resolution and grapichs with no result, the same parts dropped the FPS, so I guess is not the video card.

I dont know much about hardware, can someone help ?
That's interesting, I also have a GTX 750 TI and have the same problem.
The Rise of the Triad remake was never really fully optimized, so I wouldn't lose sleep over it being your hardware's fault (unless it's really old - but I'm running hardware much older so it's definitely not the case with that card).

Interceptor/3D Realms/Slipgate Ironworks are probably never going to issue any further updates to the game, so unless by some miracle there's a prime management of settings to tweak in the graphics options/etc. you'll have to settle with the framerate drops, unfortunately.
Try this:

It worked well on my end. Made the game more stable.