voltage fluctations

Intermittent failures can be difficult.

I would start by pulling the brushes out and examining them, not a very big job. Do the "wiggle test" on the alternator field connectors and the regulator. The regulator power, the blue lead to one field connector all are the same junction as the ignition feed, so that is PROBABLY OK

If the connections seem OK and the brushes look OK, I'd gamble on a regulator. As you may know, I don't like "throwing parts" at stuff, but a deal like this begas a little bit of throwing the dice.