The problem you're encountering is the dreaded "Mighty Mouse Scroll Problem." It comes from that tiny scroll ball getting dirty. A quick search on Google for "clean Mighty Mouse" will get you detailed instructions for getting the thing clean. Unfortunately, it's an ongoing problem-I wound up buying a Magic Mouse (which I love) because I got down to disassembling my Mighty Mouse to get it clean yet again and wound up breaking one of the flex circuits in the process.
Root cause: the scroll ball is identical in design to ball mouse balls, and it gets dirty just the same. Great idea, but the world conspired against it.