If you do what driver set do you use?
I've just rolled my back to 9.6.
9.8 and 9.9 are unusable causing glitches galore!

Gonna see if I still have issues with L4D with 9.6



Tried my old Geforce 9600GT card today, Had more trouble with them than the ati card!


SK

i havnt used ATI graphics since around 2000. tho if both the nvidia and ATI cards are playing up, it could be that the problem is not with the cards. it could be the mobo or the bios. see if theres an update for the mobo chipset available. updating the bios is a pain in the ass but check on the mobo manufacturer site to see if its needing updated.
remember you got a return to base warranty. give them a call