what about your other system components? what about your PSU? i still havent spend a lot of time using my 4850 with m2tw, just spectate a historical battle and it so smooth considering the first time i played with it was with a fx5600 > 8600gt > 9600gt > now 4850.
are you only experiencing your problem with m2tw?
i had the same problems as yours the previous month, i kept on blaming the drivers and what not, turns out its not only a busted card but also a busted crappy AIB company.
screenshot of the defective card:
screenshot of the working card:
notice the difference in memory types? a faulty bios may also be the culprit. it may sound a little off topic but its ground to have the card replaced/RMA immediately.