It's possible the switch at the helm has some corrosion and turning it off and on a few times makes a better connection.

I do know that in trying to understand what's going on I watched the amp gauge while the tachometer dropped and the output was the same. I did although notice that there was a very slight bit of screeching at the alternator when the needle drops. That screeching sound is the same as I've heard as the alternator is ramping up when first started, possibly a load thing. What I don't understand is that the output doesn't change as it's happening. When I flip the switch to turn the power off and on again the tach works and the very slight screeching at the alternator stops.

Hmmmmm?

In any event, the tach needs replacing as the hour meter on it died. This time I'm getting a separate hour meter from the tach.