[727pilot@usa.co]

You are getting stray "hash" into the line and that is what is causing the problem.

MSD addresses this in their instructions. You should be able to get a copy from their web site.

Quick and dirty try this; twist the wires that supply the tach from the ignition box. Look at your grounding. Make sure the ignition box is well grounded TO THE ENGINE. Make sure the engine ground strap to the frame is in good condition.