This is happening to quite a few people I know. Try that Nvidia Beta driver, I heard that was a decent fix. The forum replies saying it is the OS is probably BS when the very same thing is happening to many. Sounds like UT3 is a buggy POS right now.
Similar Issue
http://utforums.epicgames.com/showthread.php?t=587748
Similar Issue
http://utforums.epicgames.com/showthread.php?t=585666