OK, I'll throw my two cents in.

I have a Cateye Astrale and have seen this problem. It wasn't the magnet. It was the contacts on either the back of the computer or handlebar mount. I guess they were tarnished, corroded, or something. When the speed showed an obviously wrong number, I'd wiggle the unit in the mount and -- presto, it would start working. Of course I would forget to look at it once I got home so the problem plagued me for a few weeks.
I finally remembered to check it and cleaned the contacts on the handlebar mount and the computer with fine emery cloth and it seems to have corrected the problem.

Ron