I'm surprised that you could play at that high of res with this card...Is it Call of duty 4 or is it an older game?
Did you upgrade to that Graphics Card or did the computer come with it?
I can't think of any answer to solve this,sorry
Only thing that i can think of, if it's that old, is that it is just nearing the end of it's life span.
Maybe some other people reading this thread will be able to give a better answer.