[Ron61]

That is one possibility as when you rev the engine up it will torque a little and you could have a loose ground wire or strap that is then not making good contact. But the fact that you still read 12 volts would indicate that you still have some ground if that is the problem or the gage should read nothing if the ground was lost completely. I would check the ground wire for the gage and also the power wire where it attaches.

Ron