The regulator is not expensive and easy to change. Try it.
I see a similar issue with my Roadrunner, evening car shows extended idling, dim headlights, the old alternators didn't put much power out a low speeds and can draw the battery down.
it may charge at full capacity at higher speeds for some time to bring the battery back to full charge.
been that way since the cars were new.
Completely different than modern cars, that have capacity at idle to keep up with all the draw.