After installing the new ATI drivers (25 august ones) now I have the same problem, but even if I don't press shift or other keys.
Before I hadn't any problem.
I have Win 7 64, and a Radeon HD 48xx (4870, if I'm not wrong)
It happens with all my 3 mice: Logitech air, Logitech MX Revolution and Logitech G700
It's not only jittery, it disappears when I move the mouse quickly, in particular it disappear a lot in some screens, like the win battle screen.
The hardware mouse hasn't the problem, but I hate to use it, because obviously it does not change for different actions.
[side note: the cursor should become the standard feather when over a menu or a button]