[wwest]

Voltage measurement, and/or voltage SPIKE detection, back in the engine compartment.

Battery represents one HUGE "sink" for voltage spikes, has a very definite "smoothing" effect on voltage at the battery itself vs all the way back there in the engine/(alternator) compartment. Always clean/burnish battery terminals, battery chassis connection, engine/chassis connection, battery/alternator interconnect point at the starter 's battery supply terminal.

My experience with random over voltages in my '78 Targa, ~90,000 miles, began with the e-brake warning light latching on.

Then the engine started cutting out (rarely) seemingly randomly.

New battery, problem abated for a goodly amount of time...Targa rarely driven.

Engine cutting out randomly turned out to be related to bouncing tach.

Just a guess, but the guess is that whatever was causing the tach to bounce would sometimes "hit" the rev-limiter, which would then latch up, keeping the fuel pump disabled.

Car would not restart unless/until ignition was completely switched off, removing power from the rev-limiter, for a slow count of 10.

At the end of the day I determined that the voltage regulator was not faulty.

It was the alternator itself, internal stator insulation was mostly burned away.

Hit a bump in the roadbed, one of the 3 stator windings shorts out momentarily, VR increases rotor current to maintain AVERAGE 12 volt battery level, huge voltage spikes, spiking, back at the alternator.

CDI is "subject" to engine compartment voltages, fails shortly thereafter...

So, why does the tach bounce? The rev-limiter is a complete separate module, only commonality is battery supply voltage and tach input signal derived indirectly from the distributor magnetic pickup via CDI circuitry.

My theory...HUGE magnetic field arising, caused by, shorted stator field, coupled into the nearby magnetic distributor pickup, results in MULTIPLE firing pulses to the CDI. While the CDI itself cannot possibly respond to the burst of pulses, the tach and rev-limiter most certainly can, it appears to me anyway.

If you encounter this problem have a look at the alternator stator winding insulation.

In any case, you need a new alternator, get one with the internal solid state regulator circuits.

My CDI failed due to the extended period the internal 80 volt zener was forced to endure 14-16 volts.