Something I posted on rovers.net years ago

The Ignition Pickup Module inside the distributor (replacing the "points" on older systems) can cause intermittent starting problems as well as hesitation. When Mike's RR wasn't starting, there was no output from the ignition amplifier. (Normally, he could read 1.5 volts across the coil + and - terminals on the AC scale of a digital voltmeter while cranking the engine). A new ignition amplifier did not cure the problem, but dismantling the distributor and replacing the pickup module did. The old module looked fine and intermittently gave the correct 3K ohm reading across the output terminals, but could be made to go open circuit by wiggling the wires where they went into the rubber seating in the distributor into which the ignition amplifier plugs. Removing the insulation, he found the wires inside had snapped and were making intermittent contact. Movement of the base plate by the vacuum advance mechanism causes the wires to bend to and fro against the rubber seating, and eventually they can break. This could be the cause of a number of mystery hesitations and stumbles that people experience -- Mike was getting such behaviour as well as intermittent idling problems for a few weeks before the starting problem.