Tach sometimes works, then drops dead..help!
 

For years now my tachometer intermittently would drop to zero, then for no reason (bumps, etc had no impact) would come back to life and work fine. Recently, it worsened to the point that it was dead 90%, so I bought another one (used, but the PO swore it worked). I put it in and it's doing the same thing. How can I zero in on the problem, I don't think it's the tach anymore. BTW, it's an '84 Carrera.

Colby