W:ET FPS problem

[Conqueror]

Hello guys,Conq here.
I have a problem with my ET/PC.
It is about FPS
Well,starting with the beginning,when i join the server everything is ok,depends about which map it is.
The fps are normal at small maps like : frostbite,aldernest, etc.
Everything is ok until the map changes in a bigger one : streets of italy,baserace,venice, etc.
What should i do? I mean i am a windows 7 user,pretty shit but i think there is a solution,so if some1 could help me,i am waiting for it.