If your computer is removable, i.e. it quickly attaches/detaches from a handlebar mount, it is possible you are having problems with intermittent contacts between the computer and mount. Moisture can aggrevate this problem. I had this problem with my Cateye, mornings after about four or five miles on my commute, or in the afternoon after a thundershower on the way home the my speed went to zero and milage didn't change. The quick fix was a dab of electrically conductive paste on the contacts. I haven't had a problem since. This will only work if the contacts are a fair distance apart. You can get the paste a an automotive parts store. Good luck.