PC crashes during games only, no idea where to begin fixing

Hey guys, first post here, I'll try and be as thorough as possible.

So, my issue is that my PC is crashing after playing games for some time with no BSOD, no event logs prior, nothing. The time is always different, sometime 30 seconds, sometimes 6 hours. It doesn't matter what game, which hard drive it's installed on, or what settings I have. I've stress tested my CPU with Prime95, my GPU as well (and simultaneously) and my RAM, no individual issues. No temp issues at all. Checked all the HDDs, no issues there as well. Full malware scan, nothing. Reinstalled all drivers with a clean install, nothing. Took the PC apart and cleaned it entirely, no help. Full virus scan, no issue. The only thing I can seem to think of is that it's a potential PSU issue somehow, but I'm not sure how to monitor that or check for issues. I'm really at a loss here and I'd appreciate any help. Again, no event logs, no BSOD, just a power off and restart.

Specs are:

i5-4690k

GTX 980Ti 6GB VRAM

8GB Ballistix RAM

Z97 Pro4 Motherboard

CX750M PSU

3 1TB HDDs, 1 240GB SSD