Alternator

There are only so many causes of "fluctuation" in alternator voltage Some of them are

A bad or intermittent diode or it's connection
A bad stator. The windings can come loose and rub against each other causing internal shorting. You can see this at night with revving the engine while watching the alternator. Other than sparking at the brushes, you will see a "micro lightning show" in the stator windings

Any bad terminal connection in the charge path from the alternator output, through the bulkhead connectior, the ammeter, and back out to the battery.

it is rare BUT THE WELDED SPLICE under the dash can fail and has in a few cars.

Poor brush connections, bad condition of the rotor slip rings, grease on / in the brush holders causing sticking, etc

Especially in the old mechanical regulators, a worn out regulator

Poor grounding and or "the usual" VOLTAGE DROP in the harness. "The usual" suspects are the bulkhead connector terminals, the ignition switch terminals, and the contacts in the switch itself