KHBock:
The tach dropping to "0" is indicative of an intermitant electrical open circuit in the primary side of the ignition system IE: bad module in distributor or its connections, coil or its primary connections, or power leads and switched 12 volt circuit to the Ignition Box.
Recently I had a tachometer fail. I sent it for repair and the circuit board had failed, perhaps vibration or it was just "Time to go". Ignition still was working well.
If you haven't already, suggest you mount the MSD box on the rubber Isolatord to help eliminate vibration:
Vibration Mounts, MSD 6 Series - 8823
Trouble-Shooting - MSD Ignition Tech Support Forums