I have just recently found this problem in my video card and this might be the reason why i cant play cod4 without so much lag. The problem is, my video cards memory is taking up all the gpu clock! My default GPU clock is 400Mhz its running at 57Mhz and my default memory is 200 running at 400! I have an Ati Raedon x1200 all updated drivers.
Could be in power saving mode,when you go from a game to your desktop and view how much the card is using, the video card is displaying the desktop and doesn't require much power that might be why you see it only using that much.Also that graphics card is very weak,playing newer/modern games is asking alot of it and it cannot handle the games.
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.