Check the wiring- - - - -you're describing a classic "broken wire strands under the insulation" scenario. I've fixed at least two dozen similar problems by finding the broken, and/or frayed wires, either inside the distributor, or from the coil to the dizzy. That constant motion of the housing by the vacuum advance working back and forth is rough on primary wiring. I use high flex voltmeter test lead wire for that one, with soldered connectors on both ends.
www.ebay.com/itm/352887552664?Jerry
