Is this my graphic card or PSU: system crashing, weird restart

I'm having an issue where (mostly) when playing games (but can also happen at random just sitting there): the screen just randomly goes blank and the monitor says no signal. This happens to me when I am playing steam games. And the computer is unresponsive to keyboard and mouse. Then when I turn it off, I try to turn it back on and I can hear my power supply "popping" through the speaker over and over again every 1/2 second and the computer will not boot. The only way I can get the computer to boot is to turn the switch off in the back of the computer on the power supply then turn it back on and then hit the power button in the front of the computer...then it restarts and works normally. Other times, it was just randomly restart if I just leave it on idling after a half hour or so. I did a CPU temperature test and the highest it got was 67 degrees Celsius. Could it be the video card? power supply? It's driving me nuts. System restore didn't solve anything and I couldn't find anything but audit failures in the event viewer. Any help would be greatly appreciated! Thanks!

OS: Windows 7 64-bit
600 w power supply
video card: Nvidia Geforce 9800 gt 1gb
4gb ram