[Rick Parker]

This is in the primary side of the ignition. The electric tachometer reads the pulses created by the trigger mechanism in the primary (low voltage) side of the ignition system. It sounds like you have done well by acknowledging the "irradic" behavior of your tachometer as being an indicator. If the tachometer did not move, that would indicate another type of problem. To eliminate the tachometer itself as a potential problem, disconnect IT from the MSD (?) box or coil and see if the problem still exists. Secondly, replace the field coil in the distributor. Heat can indead cause an open circuit that will park you next to the road until it cools down. Let us know how this comes out. We all benifit from each others "Trials and Experiences". I am asuming you have checked all the grounds including the one for the ignition box itself (?)