sometimes cursor moves way too fast

I have a bluetooth Magic Trackpad for my MacPro and had the cursor speed set up perfectly. It worked great for over a year, but now it moves way too fast and is hard to position correctly. I've tried adjusting Preferences again, but they are set correctly. It seems like when I turn the trackpad off, it's normal for a minute or so, and then gets crazy fast again. It comes and goes - sometimes it's fine, sometimes it goes way too fast. I replaced the trackpad (bought a brand new one) but it's still happening so I don't think it's a hardware issue; what do I do??