Mouse scroll problem

I have an intermittent problem with scrolling. My mouse is a Microsoft Wireless Mobile Mouse 3500. It has worked fine for a year, but lately it does not scroll properly. I have changed the scroll settings from 1 line to as many as 11 lines and back again. The setting doesn’t matter. It always scrolls a half a page. I am up to date on both 12.1 and kde 4.8. Rebooting after changing settings has not helped, although it has in the past. I have tried different mouse themes with no change.

This seems like a minor thing but it is a major pain. After decades of scrolling, it is hard to remember I have to grab the scroll bar and drag it to get where I want on the page. It moves properly and all other features work as they should. Any ideas?