ASIN B00C2NQ3Q, part # 090077

Purchased Oct 16, delivered Oct 19, installed Oct 22

Installed this voltmeter on a motorcycle, with a switch on the positive lead from the battery. Initial reading looks good — 12.2v before starting, 12.6v at idle, 14.4v at full speed. Then the reading goes wonky, jumping up to 17v, flashing back to 14.4v alternately, then freezing on a single digit, randomly at any of the three digital positions. If I turn the switch off and on, the voltmeter repeats the behavior, on a shorter cycle. I changed out the voltage regulator for a brand new unit, and got the same behavior. How can I achieve a stable accurate reading?