have you tgryed to chenge your drivers what ones are you using amybe go back to using 30.82 i think they are, they are very god drivers and a lot of people seem more comfortable with them than later drivers released.
also have you checked the heat on your graphic acrd as heat can cause exactly what you are describing what fan is on the gfx card, you should check it out mate as heat may be your problem.
and also it could be your acrd itself sad to say but it does happen have you got any warrenty left on it??
but mostly check the heat on your card make sure the fan is working properly also is there good circulation were yopur acrd is sitting??
try diff Nvidia drivers do what ZIPP51 told you to and get rid of al traces of the old drivers before you download other ones.
goodlook mate
here is some good tools to help get rid of detonator drivers
http://www.majorgeeks.com/article.php?sid=806&cat=28
http://www.majorgeeks.com/article.php?sid=2753&cat=28