[wwest]

The alternator output voltage is "spiking" above ~16 volts, maybe even charging the battery to that level. The battery is electrically a L....O.......N...........G way from the alternator so circuit resistance can often be a factor.

With supply voltages above ~16 volts the CDI, Capacitive Discharge Ignition will continuously self-fire and thereby cause the tach to jump.

Another early indication is that the e-brake light will not reliability extinguish.

Once it gets bad enough the over-rev limiter will lock up and kill the fuel flow. In that case you will need to switch off the ignition entirely for about 10 seconds to allow the engine to restart, fuel flow return.