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