How are you sure it isn't overheating?
Most motherboards & powersupplies have thermal protection. Either one can cut power without warning if they get too hot.
Unless you are tracking temps you can't be sure it isn't overheating.
It is likely (most likely to least likely)
a) overheating
b) faulty powersupply
c) damaged components (the suggestion above to visual inspect motherboard and powersupply for blown components is a good one).
To help you know what to look for:
Leaking cap
Bulging cap
Blown cap