Re: bouncing tach
Depends on your pickup/sensor/tach drive method.
Tachs are usually driven either by an alternator/regulator output or by pickups mounted at the flywheel.
Have you noticed your charging system falling off at the same time? That would point to a developing alternator problem. If the tach is not alternator driven then it's possible the speed sensor connections are failing, or the sensor has vibrated out enough to lose the required proximity to the teeth or slug that drives it.
I'd say a connection somewhere is the most likely culprit unless there are other alternator related symptoms.
1984 Fast/Nicholson 345 "FastForward"
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Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)