Mysterious Self Scrolling Mouse/Web Page Issue

I've been using a Logitech MX518 for years with my iMac, using USB Overdrive as the driver. Never had a problem until a few days ago. Now, with no user interaction at all, I find that web pages will suddenly scroll down to the bottom of the page. I'm using Firefox 3.03/OS 10.5.5. I can't say that I recall seeing any issue with the pointer itself. I use Firefox exclusively, so I'm not sure if this occurs in any other browser or app. Any suggestions as to where to even start looking? Thanks!

Edit: I should mention that I'm using the same OS and browser version on my MacBook, and I don't see the problem there. Not using a mouse with the MacBook, which is why I suspect it might be mouse/software related.