horrible fps problem

i have just started playing GW again and whenever i play for about 5 hours straight the game starts getting 1-6fps which is no where near playable and at first i thought this could be a video card problem or somthing else to do with my PC. but after a while i started playing the game in a window and 800*600 and when ever i would start getting low fps i would open up RTHRDIBL which is very hard on your graphics and CPU but to my surprise i was getting 40fps in that which is normal with nothing else running but for some reason i was still only getting 1-6fps in GW.
any help resolving the issue would be great
i love the game but it just pisses me off when stuff like this happens to a good game

PC specs: CPU=P4 3.2Ghz GPU=Nvidia 6600GT RAM=1gig XMS Corsair Mobo=Gigabyte GA-8IPE1000-G HD=120gig 7200rpm western digital 8mbs cache

Does it decrease in performance slowly, or do you hit a sudden wall of bad performance?