I'm assuming your HEI is a vacuum advance unit. If so, the pickup coil is attached to the advance diphragm and rotates on the distributor housing whenever the advance changes, just like the breaker plate in a points type distributor. Since the pickup coil moves everytime the advance changes it flexes and workhardens the two wires leading from the coil to the module. Eventually one of the wires will break inside the insulation and cause an intermittent open. A quick check is to use a hand vacuum pump to advance and retard the timing with the engine idling while listening for a miss. Another check is to unplug the pickup coil from the module and ohm check it while wiggling the two pickup wires. If it fails either test it's time for replacement, which requires removing and completely disassembing the distributor.
John
