Unwanted Restarts

[Xeruan]

My old desktop, one that I am fixing for my mom restarts for no reason. I have had the computer for a 2 years, and up until about January of this year, it ran great. Now though every once in a while, mainly when the computer goes under load, the comp will restart. I thought it was a PSU problem since it happened when the PSU fired up, though after replacing the PSU it still does it, any ideas about what is wrong and how to fix it?

 

[Aaron_ATX]

First clean all the dirt/dust out of it

Examine the board for leaking capacitors ( cylindrical things on the board, look at the top, if its bulging out or busted open, its screwed )

Ram could be going bad, run memtest386 for a few hours.