TBH doesnt sound like a hardware issue to me... If it was a duff GPU you'd likely be seeing display problems, random crashes when you put load on it or no display what so ever.
Try.... CTRL ALT DEL and check the Appilcations tab, close anything there that isnt vital. Then goto the Processes tab, unless you know what your doing dont start randomly closing stuff. The 3rd row in shows CPU usage, have a glance at that and see if you've got something in the background taking a large chunk of CPU time. Also if you see anything there that doesnt look right, run a Spyware sweep (should do them regulary as a matter of course, anyway) as that stuff will suck your system dry.
Always worth turning off the Anti virus, auto update and other crap that likes to run in the system tray if you really want or need your system to focus 100% on a game/application aswell.
Is it a new card? maybe someones done some tinkering with it? could download Coolbits and check its clocks etc...