4000Q Tach trauma

[Bob]

The tach on my 4000Q has pretty much a mind of its own....  Sometimes it
acts as a volt meter where it just reads 1800rpm all the time, no matter
what (car running or not but with key on). If I hit the lights or put a
load on it, it drops down to 1500 or so.

Sometimes it works as a book end, where it just sits on 0rpm.
Sometimes it even works!!!

I have taken the cluster apart and cleaned EVERY contact on it,
including the connectors and the nuts that make contact on the foil..
Took out the voltage regulator and cleand it as well. Every other
light/gauge works fine too!
Tips?

TIA
Bob