have you ever tried to reinstall in case something is corrupt, at 1st i thought your gfx card was over heating but you play
other games right ? so that cant be the issue, try reinstall if you have already then I really have no clue =(.
that card is pretty old tho I think 2005 ? have you looked on it recently for any overheating ?
set your pc to high performance mode in the control panel / power options.
just shooting in the dark here good luck bud.