Computer powers off while playing games then stars a reboot loop

[Lgibson]

My computer keeps powering off instantly when I'm playing video games after about 20-40 minutes. Once powered off (with no warning or 'logging off' screen, just as if someone unplugged it), it tries to turn back on, but after about 2-3 seconds just powers off again. This process repeats until I hold the power button on the tower forcing it to power off. I have checked my event logs and there does not seem to be any events associated with the issue. I have no idea where to start.

I have just formatted my hard drive and got it all back up and running, but the same thing happened again. Which leads me to believe that it might be my graphics card or PSU. I don't believe it's the graphics card because the temps are not getting high enough to warrant a shut down. It is possible that its the PSU because I bought my computer from iBuyPower and I hear they sell refurbished parts. My computer has been working fine for almost 2 years, but now it just shuts down after I start playing a game.

Windows 7 64-bit
Intel i5-2320
ASRock H61M-HVS
1 TB Hitachi ATA Hard drive
2x DDR3 4GB RAM sticks
EVGA Geforce GTX 560 ti (not overclocked)
Coonix 600W PSU

Please, let me know if I need to provide any more information.
I appreciate any help. Thanks in advance.

 

[Lgibson]

Please, I have no idea where to start.