Tachometer issue?

[Jim Wright]

Odd problem on my '91 I don't think I've seen mentioned before.  The
tach has started acting up, literally.  It will read high
intermittently, then drop back down for a while.  This only seems to
be happening (for now) when I first start driving, thumping the dash
doesn't have any effect.

This is a needle type gauge, the signal feeding this comes from the
ECCS according to the manual, which reads the crank angle sensor to
determine RPMs.  For the gauge to be reading high, it would seem that
the gauge itself would be the most likely culprit, I can't see how
the crank angle sensor could be giving a false high reading, but I
could certainly be wrong these.

Anyone have any experience with a similar issue?