It's either your power supply, your video card, or you're getting heat problems.

The first thing you should try is flipping the voltage switch on the power supply to the other setting, then flipping it back. When you push it back to the original setting, put some pressure on it. This sounds stupid, but it fixed a problem of mine once.

If that doesn't fix it, try to boot and go into bios. Check your processor temperature. Are you running an AMD or Intel processor? They run at different temperatures.

Also, you could try another video card if you have access to one. If you're using AGP (likely) you could also try a PCI card (in case it's your AGP slot on your mobo).

There's really not much else that can be determined until you try some things. Give that a go, and post your results.

What is it they say about silence being golden?