Microsoft Arc mice suffer from similar problem. After some time (usually from about one month to two years) the wheel starts to behave erraticaly, it scrolls few times faster than normally or in opposite direction. The solution I've found by googling (I can't remember where exactly) is really simple.
Open up the mouse and remove the scroll wheel. Cut a few millimeters wide strip of adhesive tape and wrap it around the tightly attached end of wheel's pivot. You want just to make it a little bit thicker, so don't use too much tape; one layer should be enough. Reattach the wheel. In case you can't put it back in its place, remove some of the tape until it fits.
It worked perfectly for my MS Arc Mouse, the wheel problem disappeared completely and it was fine for 8 months or something before I replaced it with a new one because of different problem.