I've had the same problem with my Cateye. The mph's bounce all over the place, and I know from comparing other computers that it doesn't record the right milage. It seems to freak out the most when it's hot and humid. I've played with the magnet and sensor positioning. I haven't changed the batteries (it's not that old, though I know that doesn't really mean anything). I'm tempted to chuck the thing and get a new one. If anyone has any ideas...