Simple wireless computers do that in the presence of EM fields from power transformers, overhead power lines, the eddy loss sensors used to trigger traffic lights, industrial equipment, some LED head lights, etc.

Upgrade to a better wireless computer which has sensors that speak the digital ANT protocol where the worst that will happen is a drop out (very rare at 2.5GHz vs the lower frequencies simple older computers used) instead of ludicrous readings that throw off your totals and averages. A few hundred dollars on a Garmin Edge 200 or 500 should do the trick nicely.