Graphic Issue

Hi, I have just got some trouble with the graphic in game.
It keeps 10-15 fps only with the Grundorf city and worse when I have played for longer.
I have tried to restall the graphic card driver and the problem can be solved. But it becomes laggy again after I have played for longer.
Does anyone have any idea about it?
* I found that the CPU keep loading at 5x% or even higher, is it related to the problem?

Here is my PC's config:
CPU E6850
RAM 4GB
Display ATi HD4650 1GB ( a bit old )

Thanks!