PUBG restarting PC

When loading into a game or after a game ends, the PC restarts. It's running a 4790k and a 980 ti and everything is stock. The PSU is a 650 watt EVGA supernova Nex, gold rated. I've checked everything that I could think of. The temps are fine. Under simultaneous stress tests, the GPU barely hits 70 (valley benchmark) and the CPU might peak at 80-85 (aida 64), but during PUBG gameplay, the CPU has never even hit 70, and the GPU stays around 50. I also thought it might have been the power supply having issues with the changing power state that occurs when you load in and out of a game, but I imagine the same behavior would show itself when starting and stopping stress tests, but PUBG is the only game/program that makes it restart, and the stress tests running at the same time certainly pull more power than any game will. Again, the game plays fine once it loads, it only restarts when loading into a server or exiting a server. Could PUBG cause some unique issue with the power supply that other demanding games and programs don't? I've done clean installs of the GPU drivers as well as reseating the cpu/gpu/ram (16 gb) and unplugging and reseating PSU cables into PSU and motherboard. Windows 10 is up to date. While the computer isn't new, the system is clean. It's actually my brothers PC, and he has virtually no programs on the computer apart from 5-10 games, steam, msi afterburner, discord, chrome and maybe 5 others TOTAL. For that reason, I haven't tried a fresh install of windows which may be my next idea. Motherboard is a gigabyte z97 mx-gaming 5. Its just over 3 years old, which could be the issue being that its just out of warranty. I've tried two different pcie slots on the motherboard which didn't make a difference. There's other things I've tried too which I'll update if i remember. Unfortunately I can't pinpoint exactly what PUBG or windows update this issue started to occur with, if at any particular one at all. Others have had this same issue, with no luck from their solutions. I will be trying a different PSU soon and will update with that info. The symptoms point to a bad motherboard or PSU, but only in PUBG which seems strange to me, Sorry for the long read, but any help would appreciated. I feel bad for selling my brother a broken PC!