I don't see how your tach, being a simple direct current gauge (moving field winding over a permanent magnet) with added circuitry (the rpm's which are electronic pulses converted into a constant D.C. voltage representing a value on the gauge)could be the problem. It could be your speedometer cable. Does it happen it gear or all the time? Maybe a bad capacitor could be making the gauge needle vibrate???? Don't know.....let's see what more people have to say...