update directx, os drivers, updates, & patches, and .net? fresh install of game & updates, and removal of drivers/re installation of video card? defrag hdd, are any processes using cpu cycles? do you get the hitching online and offline multi/single? do you get the same problem in another 3d shooter? have you tried lowering to lowest possible game settings/graphics/resolution to see if you still get the hitching? from this point you can enable higher settings one at a time until you find the one giving you problems. cod4 is very customizable graphically.
good luck!