If the magnet is moving around on the spoke, then that is the source of your problem. I have had the same issue. I little superglue to hold it in place works wonders. I figured that was my problem because I was getting obvious speed errors- would drop to half and then come back up, drop to 0 then come back up. My wife has the same computer, but has bladed spokes- the magnet can't rotate. We compared stats after rides together, and the difference in avg speed and distance over one of our regular loops was pretty dramatic- I got 16.25 miles instead of the 22.28 that she got, and my avg speed was a little over 11 mph, while she was at 15 and change, and we were together for the entire ride.