I'm trying to figure out why my mother's mouse is working so poorly on her Mac Mini. When she just tries to highlight a passage, the highlight will do any number of things, from disappearing completely, to jumping the selection around in the passage, to weirdly dragging portions of text out of place (this is, of course, the most annoying behavior).

I can't tell why this is happening. Any ideas? The mouse is a Logitec Performance MX.