only thing i can think of is graphics card is strugeling have a look in event viewer see if there are any errors in there maybe that can shed some light on the subject
what version of direct x do you have ?
old games and latest direct x can cause problems
also what about pc temps ?
run everest http://www.lavalys.com/products/over...?pid=1&lang=en
and check system temps befor and during the game
Try running it in 640 X 480 resolution if you are not already. Only other thing that I can think of is to uninstall your graphics card in safe mode along with the folder and contents that may be left behind after the uninstall. Re-install graphics card. Always uninstall grahic cards in safe mode before updating graphic card drivers. May not seem to do any harm not uninstalling but it will catch up with you. Games being messed up is an early sign......