Computer shuts off at Random

[Sinko44]

For the past few weeks I have been having a terrible time out of this computer. It all started one night when I was playing World of Warcraft. The computer just shut off as if I had pulled the cord from the power supply. Therefore, I started to troubleshoot the problem. I decided to take all the parts out and run it on a bench. The only hardware I had running was the motherboard (of course), both sticks of memory, and power supply. It would shut off within 30 seconds. So I came to the conclusion that it was the motherboard. I purchased this ASRock motherboard and just installed it yesterday, but the problem persists. The weird thing is that I can run the computer fine for hours sometimes and it WILL NOT shut off. Then, at a random time it wil decide to shut off and do it over and over again. You can go ahead and rule out an overheating issue because it will do it when I first turn it on after sitting off all night. Plus, I keep my computer dust free. Also, I went ahead and ruled out the power supply as the culprit. I borrowed a brand new 500w power supply and hooked it up....same problem. I also tried each stick of memory seperately while running the computer and the problem still occured. Heatsink is clean of all dust and is properly mounted with arctic silver 5 thermal paste. Any help/suggestions for the possible cause of my problem would be much appreciated.