Computer Shuts Down automaticlly

Hello, My names Cj, I have an issue that I can't figure out and need a bit of help with. Periodicly the computer will shut itself down, no warnings or anything. After it turns off and I hit the power button again it doesn't start, I have to turn the powersupply switch off then back on for it to start again. Any suggestions?

Asus M3N78 Pro MOBO
AMD Phenom 9550 Quadcore Processor
Asus Silent Knight Heatsink
2x2gb OCZ DDR2 PC2-8500 / 1066 / Platinum Series / Low Voltage / 4GB /
500gb WD Caviar Green HDD
Coolermaster Case
600W Power Supply
Windows 7 Ultimate