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My experience has been that a cold out inconsistent tach was the tach itself. Hook up a timing light to the trailing ignition when it is not working, or pull the trail coil wire, making sure you are not grounded to the car. If you have spark, you have a bad tach, if it does not, check wiring.
Igniters usually are not inconsistent failures they either work or they don't. Out of the hundreds I have diagnosed I think there was one or two that were.
Bad grounds, broken wires or connectors are the most common failure issues in that order. Just because a wire looks good does not always mean it is. I have found many a broken wire under good looking insulation. Continuity test was the easiest way to find them.
Without being able to put my hands on it and from the symptoms you give my money right now is on the gauge itself.
Other issues that can show at this age of the platform would be with the circuit board. Rust, corrosion or damage circuit.
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