Since you replaced everything that would be a possible cause for a erratic bouncing electrical tach needle, and i'm guessing it's working ok at idle but it's dancing at high rpm as if your ign point set was floating?
I would try another ign coil just to see if it's not the secondary windings breaking down and causing the tach flutter even though your engine is not noticing it, if not that then the other possibility would be a worn distributor shaft but then you would know it with a wobbling dwell angle while setting your points and the engine would be barking at you instead of running fine?
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