Alternators need a battery to stabilise the output.
You said that the voltage is all over the place. All the alternator circuits that I've seen, including this **broken link removed** have the alternator connected to the battery. Now if the voltage is changing on the alternator, either the battery voltage is changing or there is some voltage drop between the two.
The only thing in between should the ammeter. What is the ammeter showing while the voltage is varying?
You could try bypassing the ammeter and seeing if the voltage stabilises.
It could also be that there is just too little electrical load on the system. However, there were alternators on cars from about 1970, and cars of that age had just about no electrical load when driving unless something was turned on.
I converted a car (well a Reliant Supervan III, so nearly a car) from a dynamo to alternator, and I fitted capacitor discharge ignition, and had no problems. However, it was 12 V to start with and it didn't have an ammeter. The only electrical load that was always on was the gauges, but I probably left the heater fan on most of the time.
The electronic ignition is likely to reduce the current taken by the coil, especially at low engine speeds. I would have thought that the battery would absorb the minimum current that the alternator produces, but you could be overcharging the battery.