Magic Mouse is slow and jittery frequently

I bought this Mac Pro about 2 years ago and right away I noticed that the cursor using the wirless magic mouse was not smooth. Of course, it worked fine when I went to the Genius Bar, and I eventually bought a Belkin USB/Bluetooth thing that I stuck in my keyboard and that seemed to resolve the issue.

I've recently relocated my setup, and the Mac Pro is actully much closer to the mouse and even with the Belkin USB/Bluetooth device plugged in, my cursor is frequently getting slow and hard to deal with.

I'm wodering if anybody has any fixes for this.

Thanks,
Justin