Sound like the PSU to me more than the MB. Okay once you unplug the unit for a few minutes will it run fine, for a while, or do you get maybe some screen flickers, and other odd behavior the longer it runs? With every computer I have ever owned I have gone through more PSs than MBs. Except for when I was Over CLocking machines, then that was from pushing it. Oh look and see if your PSUs fans are running after warming up and sniff for any odd smells like toasty electrical, not burning but toast. Power supplies can sometimes drag on and cause minor little problems till they die, or go all at once. THe only way to be sure is using a Volt meter but, I am guessing you do not have one.