Hi Lee, Yes you are correct that the whole electrical system (minus the starter motor circuit) goes thru the ammeter and therefore makes it a single point of failure. My gut feeling is that the rev levels that make a difference is down to vibration and some levels of vibration the circuit "likes" and others it doesn't. To me, most of this points to a loose connection somewhere on the distributor feed circuit, and most probably between the battery and up to and including the ignition switch. Possibly the engine ground strap too, or the main battery negative lead. If it was a bad coil only, the ammeter would not be hugely affected.
With the cleaning job on the ign switch, has the fluctuations on the ammeter stopped? If so, I'd say you have a separate lesser problem with the carb settings.