Lummer:
Once again, for what it'd worth, the Green Book for the 1988 760 reads as follows: "revolution counter: The rev counter shows the engine's revs (yada).
The counter receives its signal from the ICM, 2 pulses/rev.
The ICM receives information on the engine's revolutions from the RPM sensor mounted on the flywheel.
If this applies to your machine then there might well be a problem at the source of the tach information.
As well, I searched the forum for "740 tach" and found an interesting entry from "Evinrude" which you might read, just for a different approach.
Anyway, keep us informed - I'm still waiting for my instrument suite to go boom one of these days, judging from the volume of traffic on the subject.

gus_boy