T&T: Tachometer Problem

[GARY RITZMAN]

One of the VDO tachs for my Lehman 135s seems to "skip" or "bounce." For
example, running at 1700 RPM, it will drop to zero and then immediately bounce
back up to 2000 before settling back to 1700. I thought that perhaps the belt
was loose but that does not appear to be the problem. Any suggestions from the
fertile minds of this knowledgeable  group?
<<<Gary Ritzman>>>   M/V Dharma   Albin 40   Mercer Island WA