If the system is a year or older you could try replacing the battery on the board as the low cost method to see if the original is weak and about to go. I've seen that one a few times. Without being on long high temps from something overheating can be ruled out except if some type of short to ground was being seen pulling the power down.
A bad cap in the supply, bad memory, problem caps on the board itself are the typical faults seen. Is this on a prebuilt system like a Dell, HP, Gateway, etc. or on a custom build?