hunter315 is right, sounds like your GPU is overheating or your PSU isn't quite up to the task of providing the needed power to your GPU. If you are overclocking your GPU, stop and revert to default settings to see if that helps.
If that does the trick, check the cleanliness of the inside of your system (remove any dust bunnies and hair balls, etc) and make sure all of your fans are indeed turning properly. You should do this anyway, but definitely before attempting to OC.
Also, make sure you have good airflow, in general. Move/re-position cables, etc to the max extent possible to have air flowing through the system as unonstructed as possible.
Hope these pointers get you in the right direction. HOOAH!!!