scrolling by itself

For a few months I have been having a strange phenomenon. My pages scroll by themselves without touching the scrolling ball. I thought the mouse might be dirty so I cleaned the trackball (wired mighty mouse) but that didn't stop the problem. I bought a new mouse yet the problem persists.

Sometimes it happens when I move the mouse without touching the scrolling ball. Often touching the ball can get the scrolling to stop.

Any suggestions?

RS