Mouse Cursor "Stuttering" on Web sites with animations

The last few days I've had an intermittent problem with the mouse cursor moving erratically when I am on a page that has an animated banner, especially if it's near or on the right scroll bar of a page. My OS is Windows 7 - 64 bit and I have an old-fashioned roller ball mouse connected with a PS/2 port. I already checked the mouse for internal cleanliness, and it it remarkably clean. Why is it now moving intermittently like this?