As suggested above, make sure your GFX card is firmly in place and that its temperature is within the limits.
If you are having a problem with the GPU temperature, you could get a better cooler fan for it.
Also check the quality of power supply, insufficient or poor quality supply can also cause problems with your GFX card.
Finally, let's not rule out the possibility of the GFX card showing signs of failing.
Hope this helps.
Edited by ReviverSoft, 17 December 2009 - 02:36 PM.