Sounds like misalignment somewhere--in some cyclocomputers, the magnet has to pass very near the pickup, too: so a bit of wobble could cause it to miss now and then.

With wireless computers (I don't know if yours is or not), radio frequency emissions, especially in industrial districts, sometimes cause misreadings. This happened to me once, but only once, with my Cateye wireless.

I would put the bike on a stand (if I had one) or rig it up somehow so that I could turn the wheel by hand for a while and check things out.

Good luck....