pc turning off and on in game

[gm9]

I agree, seeing as it only happens when the system is under heavy load this is likely a hardware issue, and I would agree with the assessment that it's either overheating or your power supply. Overheating can be simple things like just your memory sticks overheating, and you may not have fans blowing on those. And even if you have a 1000W power supply it may still be a bad one that delivers fluctuating power levels which will cause general system instability. If you can just swap the power supply out for testing purposes and see if it helps. If that is not an option start disconnecting hardware like optical drives, PCI(e) cards, etc., to reduce power consumption and see if it runs stabler. If that helps you will still need to get a new PSU though.