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I had a similar problem years ago and it was a bad connecting rod.
Good news: the tach needle stopped bouncing.
Bad news: the rod went through the block.
Remind me to never let you drive one of my cars!
I agree with Steve that it's a connection that's causing the problem. Interrmittent ground or the trigger signal from the coil. As long as the engine runs smooth and strong, it isn't anything in the ignition system. If you re-establish the continuity of all the tach connections and you still have the problem, the tach is broken.
Verne
