I think it could be an overheating problem possibly. To discriminate between a GPU problem and an overheating problem try this...
1) Download
Prime95 and run the torture test for 24-48 hours. This will bump your temperatures up, and give your pc a hammering. If it has no errors and your computer doesnt reboot...
2) Download
3D Mark 05 and this will thrash your graphics card.
Try updating the drivers first I suppose. But if either of those two programs make your computer reboot (esp. Prime95) I would be guessing PSU. How many fans does your computer have?