Help with a miss and voltage drop

"Some things" I would do. If your voltmeter is routed into the engine bay, temporarily rig it so you can monitor various different points. Probably the first would be the IGN1 (ignition run) wire right near where it comes out of the firewall into the engine bay.

Another thing to try is to run the car a short ways (daytime) with the voltage regulator unplugged. This will eliminate the alternator as a source, and if you are losing battery power it might show that up. Try to drive on low traffic, county, etc roads.

With the engine idling, go around and wiggle the livin' 'ell out of a few things.........the harness where it approaches the alternator, the bulkhead connector, and around the distributor/ ignition. Also around the starter relay. See if you can duplicate the problem