I helped build my fiancee's computer.
Any graphically intense game will eventually (usually after about 15 to 30 minutes) will crash her computer. And by crash, I mean both of her monitors will go black and not display anything. You can still hear the game sound in the back.
Here comes the strange part. If we hard reset her computer right away, nothing will display on her monitors, not even the BIOS POST or anything like that. We can hear the windows startup sound when Windows loads. Sometimes when we wait a few minutes and hard reset the computer, the monitors will start getting picture again. Other times we have to actually turn the power supply off and on in order for her to get any picture.
She has a R9 390 video card.
Things I've tried:
I have tried stress testing the computer with 3DMark, and it ran the tests for over 2 hours without crashing. (I don't know why games cause crashes yet a stress-test that brings GPU usage to 100% doesn't)
I've checked the temps of the CPU and GPU and they are both within well acceptable levels. (about 40 for the cpu and before a crash the GPU was anywhere from 50 - 60)
Ran a Memtest overnight with no errors
I've uninstalled and reinstalling the graphics drivers.
Games that have crashed her computer so far: Cities skylines, Sims, Skyrim