Okay so I am at a loss here.

My brothers PC keeps randomly restarting, like powers completely off then turns itself back on. It doesn't matter if he is playing a game or watching a video or even just sitting on the desktop. It also varies when it does it, like he will get a few hours gaming and then the next he just googled within the first 5 mins of a restart. We have tried various Troubleshooting methods including part replacement.

Things we have tried.
-Changed power supply
-Changed Motherboard
-Reseat RAM
-Taking out one ram stick at a time to see if we have a faulty one(Rebooted still)
-Switching PCI-lanes on GPU (more on that in a moment)
-changed CMOS batter
-Default BIOS settings(no XMP and any other settings to see if that was it)
-EXTENSIVE thermal monitoring
-Re pasted CPU
-Clean Install Windows
-Changed Outlets just incase
-COMPLETELY reassembled the Tower numerous times

GPU: GTX 1080
CPU: Intel 8700k
RAM: 4x8 Corsair Vengeance 3200mhz
MOBO: ROG Maximus CODE(old) and Some ASUS MOBO for Testing(Brand new bought, just cant think of the model)
AIO: Corsair 1150i (or something like that)
PSU: First one Corsair 750 Watt the new one we just bought is an EVGA 750 Watt
--Idk if im forgetting anything if so let me know

So on the ROG MOBO the POST code kept going between many different ones but kept landing on 7F which from my understanding is a video error but i do not see how that would randomly shut down the pc and then restart itself, maybe loss of video but I guess I could be wrong.

Idk maybe i am missing something Please HELP! also sorry if this is hard to read