GT40 Pro Boss tach problem

Luciano I just put some vintage gauges in my boat and have a similar problem.Mine is Teleflex so I looked them up and ran across this, maybe it will help you too.

Bouncing, Erratic Tachometers.

As analog tachometers age they sometimes begin to behave unpredictably and do not always display the correct RPM. Sometimes tapping on the glass brings the tachometer back to the correct reading…temporarily.
This abnormality may be caused by a thin film that grows between the Selector Switch contact and the circuit on the printed board. This film tends to intermittently insulate the two causing the strange readings.
To remedy this, simply rotate the switch about 6 times, resetting the switch to its original position. This should eliminate the film build up and all return to normal.
Erratic operation can also be the result of old age or a power surge (lightning for example) acting on the internal electronic components. This you can not remedy and the tachometer must be replaced.

Here are more questions the answer on all their products.

This one does not respond to glass ticking.