FWIW I just changed out my Powermaster alternator (I think it was a 85 amp one) for a new 125 amp Powermaster.

My car was doing exactly what yours is and finally started trying to charge all they way up at 16 volts...Lights dim at an idle but everything worked fine once off idle...

With the stereo load I have when it is turned up I know the 85 Amp didn't keep up with the draw...I bolted the new 125 amp alternator on the factory brackets after making all new spacers on my 383.

It's a world of difference...no more low charging at idle as the gauge reads a consistent 13.8 all of the time...

The added amps allow me not to drain the battery while driving with the music cranked up...

I think the bolt on voltage regulator on mine was bad since the alternator does still charge...That may be your issue also...