Hi Paul, If the gauge is otherwise working properly it's worth checking the integrity of the ground connections in the harness, ignition system and engine block, would be a shame if you bought a new instrument and it did the same thing.
Spurious noise from the harness can effect the gauges... tach, volts, alt input wires to gauges could be suspect.
Something else all GT40 replicas should have is a master battery disconnect switch that shunts to ground on shut off. Can also save your ignition box or other onboard electronics.
Are there any markings on the back of the gauge to identify the manufacturer? Could be a VDO. Customer support at Caerbont and VDO/Siemens is excellent, you'll get it sorted