Could be CPU protection against overheating - if your motherboard has one - but as it runs "for hours" afterwards, probably not. (Unless, of course, it only happens when the CPU fan switches off). Your computer would not start again until everything cools down.
Personally I would invest in another, bigger, power supply, but that is only my opinion for what it's worth. If all fans switch off, I wouldn't take the risk of fatal damage for the price of a decent power supply.