Problem with my graphics card??

Problem: system becomes slow and game gets stuck continuously after a few minutes of playing CoD MW 2, and continues to be slow for 10 minutes after I quit the game. No doubtful processes seem to be running on the windows task manager.
Duration: Since few months
System configuration: Dell Studio 1558 x64-based PC
Processor: Intel(R) Core(TM) i5 CPU M 430 @ 2.27GHz, 2267 Mhz, 2 Core(s)
Rams: 1x 4GB DDR 2
graphic card: ATI Mobility Radeon 5470 1 GB
Hard drives: 500 GB ATA Device

What should I do? Any diagnostic help/solution??