Computer completely restarts while playing games?

**Parts List: PCPartPicker part list / Price breakdown by merchant

Problem: Bought and built this computer Black Friday 2015. A couple months ago around late October, my computer just began cold restarting after about an hour of playing games. No warning, no error messages or blue screen, just total power cut for a split second before seeing my motherboard splash screen. I initially assumed it was just one game and then realized a couple weeks later (I was very busy) it was all of them. I assumed it was my power supply, due to the nature of the problem. So I called EVGA, just to ask about warranty information, and he assured me my warranty was fine but asked if I checked the video card, which I did not think could cause this problem, and told me that was more common than I thought. So I convinced my roommate to let me borrow his 970 FE, played a game, everything worked fine. But the problem wasn't consistent enough for this to satisfy me so I tried a bunch of benchmarking programs until I found one that produced the problem consistently, which was OCCTPT. When I ran the GPU test for more than about 60 seconds and tried to alt-tab out of it, the system restarted every single time. I borrowed my roommate's again, did the same thing giving it more than enough time, no restart. Conclusive for me. So I put off fixing it until winter break, and ordered an ASUS ROG Strix 1070 (didn't want to deal with the bullshit of MSI customer service after reading a bunch of horror stories and having no luck in contacting them myself so I decided I'd buy a new card and then RMA the old one and sell it) which arrived today. Popped it in my computer, played Overwatch for about an hour, restarted.

Can anyone explain this/ anyone else experience this? It's absolutely maddening at this point. And how can I further narrow the problem down? How could I test my motherboard/processor?