Tachometer bouncing (MSD ignition wiring question)

Download the appropriate MSD wiring diagram for your model and recheck/redo it all for the distributor you are using.

Sometimes MSD doesn’t play well with factory tach’s because of the ‘multiple spark’ thing. Tach adapters are often required, however I would try connecting it to the negative side of the couple first, then the dedicated tach feed off the msd before buying an adapter.

Good luck