How to improve my fps?

Hello everybody, I have a problem here ...
I don't know what is happening, but when I get close to other player during the game my fps drops, gets to go to 5 FPS unplayable. My card is an ATI HD 5750 Vapor-X is not likely to lag. I play PES 2011, 2 Modern Warfare, Halo 3,without problems with FPS, but my fps in Crossfire is low even with the textures in the "Low", and it happens that I mentioned above.

Sorry my English, i don't speak verry well. =/

Thanks.

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