Steve you can make some simple tests right on the car. That is for certain the older 69/ earlier setup with a replacement regulator
I would pull the brushes and examine them and the slip rings, for dirt, grease, how worn (short) they are. They should be under spring tension
Rig a jumper wire from the battery to the field connection on the alternator. Run the car while watching battery voltage on the meter. It should start to climb, but don't let it go much above 15.5 or so, control with throttle. Turn on headlights, re-adjust RPM it should "keep up."
If it charges unsteadily it might be a bad brush, brush connection, or might be in the wiring path between alternator and battery. It could also be a stator "rubbed raw" (insulation rubbed off) as well as a bad diode. This is inferring the intermittent condition.
To eliminate the charge wire path, jumper a heavy wire direct from the alternator stud to the battery. Repeat the test. If the intermittent goes away, check the output wiring.
If it still jumps, comes, goes, or won't charge, time for another alternator
