Vertical Scroll always goes back to top of page

This happens to my client several times a day. It is resolved by exiting Notes and logging back in. He is in Notes 6.5.4. This does not happen in any other Windows application. Mouse drivers are up to date. When he scrolls down the screen in Notes or in IM, the vertical scroll will automatically jump back to the top and he cannot drag it back down. Scroll Lock is not on. I appreciate your help.Melissa