Don,
Start at the connections on the Master. Make sure clean and tight... Radio Shack sells some stuff call "De-Oxit" and it cleans at the molecular level. Take the connections off and use a Dremel wire wheel and spray it with the DeOxit. If it was working normal and you had a slow degradation, probably resistance "somewhere" on the connection path...
Not 100% sure on the elastomeric connection of the alternator, but it either works or it snaps... You can check by looking in the aft portion of the Alternator and rotating the prop to see if there is any slop in the movement.
One other thing to check is if the voltage changes with the load. Turn everything on in cruise and watch your amp meter when you key the mike, you should see a dimming and or good discharge, but volts stay the same as you are pushing the charging system to the max...
Let us know how you make out. Also, if the regulator was ever changed, it might be installed where your Loran or ADF used to be????
Take care,
TMcD
1980 M20K