+1 on wired computers. Cat-eye's Mity 8 and Enduro 8 (same head, Enduro has a heavy duty wire) are wonderful wired ccomputers and now at close-out prices since they've been replaced with newer models. Buy two and never have this problem again.

Wireless devices, particularly ones that aren't properly "coded", are prone to all kinds of odd-ball interference. A friend had heart surgery several years ago and bought a cheap Nashbar heart monitor to assure he didn't exceed his doctor's advice on maximum heart rate while riding during his recovery. We rode together the first time he used it and as we rode past a power company transformer station his monitor jumped to 250 bpm and then to zero! Talk about scared! It took a moment for us to realize what happened and the reading went back to normal a few blocks later. He later bought a Polar ($$) coded monitor and that ended the interference problem.