Like others have posted, it could be motherboard or ram, or even powersupply.
Personally, I would start with the power supply. That is the first entry of any electrical disturbance and the PS could have taken the hit or whatever spike you may have had. Plus a power supply is easier to swap out than the motherboard. Of course RAM is the easiest. But the P/S is probably the cheapest, and you might even have one lying around that would fit.
I had a similar problem and swapped out the P/S and it worked fine after that. Something in the P/S was failing after being on for a while.
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