Mouse pointer does own thing

I am running Windows 98 on a AMD K^2 500 machine that has 128mb of RAM. Recently I have been having problems with my pointer. I can move the pointer to the bottom of the screen and leave the mouse alone for some time. It would all of a sudden jumpback up to the top of the screen(like it is possesed). This happens constantly and happens more frequently while using it more.
Initially I thought it was a mouse related so I cleaned it and re-adjusted the configurations. Then I thought it mightbe video card related, so I lower the use of the vcard. Then I thought it might be a virus so I updated the definitions and ran a scan.
It was none of these. Please help.