5870 sudden FPS drop in MW2 and BBC2

Hi, Good day all,

I just wanted to ask if anyone has an idea why im getting low FPS when im looking at dense areas in BattleField Bad Company 2 and MW2

Just last week i was playing these games and got fps around 80+ up in all areas in mw2 1920 x 1080 full settings and in battlefield badcompany around 40 to 50 fps 1920 x 1080 full settings. Then when I played just yesterday, MW2 get an fps drop to 30+ when im looking at denses areas and returns to 80+ when im looking at a wall. In Battlefield Badcompany 2, i think my FPS no dropped between 25 - 35.

My specs are as follows
AMD phenom II x4 955 BE 3.2ghz
4 gb dd3 ram
500gb HDD
800w Odin Pro
ATI 5870 1gb

Thanks and hoping for some answers..