I would look closely at the video card. Start with a new driver. If that does not work, lower your resolution/color bit size/hardware acceleration to see if it runs a little more stable.

If all else fails, you may need to try and replace the card.

In addition, you can check for shared IRQs. Too many devices on one IRQ can cause these types of problems. Specifically, I would look at the video card.

The easiest way to fix an offending IRQ problem is to take the device in question and move it to another PCI slot farthest away from the original.

How about your power supply? Is it adequate for all the hardware that is running on your computer (USB, FIREWIRE, extra devices)?