Used to run COD4 fine on same computer with same 7900gs graphics card. Now, after couple of minutes, get various colors/lines through the screen, blocking some of the area. Have upgraded to latest patch.
I had a 7900gs as well and ran COD4 with no problem. Check the temps it reaches under load. If they are high that's probably the problem. Else you GPU could be dying.
Also, did you overclock the card?