Here, generally is what the problem might be, in no order:
1....As I mentioned, a poor designed voltmeter
2....Voltage drop in the harness, which can include bad connections of any kind, in the bulkhead connector, the ignition switch connector, the switch contacts themselves, or partial failure of the big master splice in the black ammeter wire
3....Poor grounding especially the VR
4....In rare cases a bad VR
5....In rare cases something with the battery, perhaps sulfation.
I assume you have a mostly factory harness, and still have the bulkhead connector. Voltage drop from several sources, and sometimes a combination of them, can cause this. With what you describe, if it is NOT a bad voltmeter, you should be able to "see" this in the headlights/ or dash lights with the parking lamps on
One thing I recommend, because of voltage drop problems, is to add a relay to operate ignition "run" buss loads which then gives the VR a stable voltage.
Electrically cut the ignition 1 "run" wire coming into the engine bay out of the bulkhead. This is NOT fused originally, and feeds the ignition system, VR, and alternator field. On some cars it feeds electric choke and maybe some smog doo dads.
Take the bulkhead end of the wire and use it to trigger a bosch relay. Feed the relay contacts with a fairly large, IE no 12 ga through a fuse / breaker from the start relay "big stud." Connect the load contact to the engine bay harness end of the "run" wire you cut.
Also make REALLY certain the VR is actually grounded. I like to run a big jumper from the engine to the body, and in original form, this is pretty minimal.