[Bob Putnam]

Is the car running smoothly for those 5 seconds, or is it stumbling badly? The latter might indicate running on from pre-ignition - which might have been caused by a carbon build-up from the previous bad running.

Disconnect the voltage regulator plug to see if that's related to the run-on. I would replace the regulator anyway, after checking the voltage with another voltmeter.

If the engine is after-running smoothly, you may be getting feedback through the alternator, possibly related to the accessory terminal on the ignition switch still being live. Try disconnecting the wire from the accessory terminal to check that. If you have an electric fuel pump, you'll have to run power to it.