Chances are you have a multicore CPU. Its an old issue with DE:IW, there are two workarounds that I know of, one is altering the affinity of the app so it only utilises one core (task manager); the other workaround is disabling every core on your cpu except one (there are startup switches for this). Have a search around the old eidos support forums for this game.
-- INTEL 9xx SOLDIERS SANS FRONTIERS
