One other consideratiion. In distributors where the vacuum advance moves the pickup coil back and forth to vary ignition timing according to load conditions on the engine, those tiny wires between the pickup coil and the module are in motopn constantly. Strands of the wires under the insulation can break, and cause an intermittent open circuit. That's also difficult to find, but one diagnostic procedure involves disconnecting the vacuum line to the advance unit. If the problem goes away, I'd strongly suspect broken wires between the pickup and the module.
Jerry
