could be youv overclocked your system to much as well and it's overheating although i think youv prob just overclocked your card too much, post your system specs if you want better answers.
What gfx are you running Ive got a 9600 clocked at 505/303 and 2001runs perfect as long as i turn ati tools (the oc stays) and my bench marks jump from like just under 10000 to 13500. I suggest trying to turn at tools off.