Why don't you connect a temporary harness from the tach to the distributor and ground and see how it works? That will tell you if the tach itself is bad. If it doesn't misbehave with the temporary in place then the harness is bad. If it still misbehaves, it's either in the HEI or the tach itself. If you have a spare tach around you can hook that up to the temporary harness to determine if it's the tach or the HEI.
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