Hmmmm...
If you can, I would get a multimeter from someone and check the power supply.
Even though it sounds like a power supply problem, it could also very well be power on the Video Card since it's only with games.
Also, things like this have been known to happen when the onboard video is on at the same time as the Video Card.
One other thing is flashing the BIOS. If the BIOS is out of date, that could very well be the problem
Hope some of that helped
~snake~