I had a similiar problem with my expensive Intellimouse explorer. It was starting to really anger me- so I took it apart. Turned out some crud was stuck in the groove between the mouse button and the case itself. So I scraped a pound (ok, exaggeration) of dark coloured crud (dead skin? walrus roe? who knows?) out of it, slapped it back together, and good as new! (well, aside from the black mark on the left mouse button right where my finger sits...)