I've had this problem since i bought my PC. I have an FPS Issue with older clients, Not sure what client patch it starts to work but its around 5118 I think.
What happens is my mouse flickers between the conquer mouse and my mouse - The default mouse... It must do this and flood my computers processor. Some clients used to just crash until i added it to my Data Execution Prevention.
Additional info:
When the mouse is still i get the full 39/40 FPS. As soon as i move it it drops to 9-19.
I have a dual core 3.4ghz processor - :/. + Sometimes a 512mb graphics card but usually intergrated intel hd.
Any help is appreciated.
Thought i'd mention i'm running 64 Bit Windows 7.