I just installed an ATI HD 4870 card to solve a problem of HD video not playing smoothly on two monitors with the original Nvidea GT120 card. Now I have a mouse that stutters and freezes all over the screen.
I see on the Apple support site this isn't unique to my problem but seems to be an issue with certain monitors attached to the DVI input.
I am running two Samsung monitors, DVI on one and Mini-Display on the other. So I started swapping monitors. Samsung on the Mini-Display was fine, Samsung on the DVI jumped. And not just the mouse but the keyboard too. Since they run the USB cable through each I think this is a USB issue when the DVI plug is used with a DVI monitor.
I ended up using a VGA cable with DVI adapter on the computer and VGA to the second monitor and it works fine. Straight DVI didn't.
Kind of ironic that I upgraded for working in HD digital video and the problem is resolved with an analog cable.