69 Dart GTS 383 4 Speed Completely Original

I had something similar, and it was indeed the alternator HOWEVER when I replaced the alternator I continued to have the same problem which sent me down different roads - but it ended up that the alternator I bought was dead in the box, and as soon as I tried another alternator, the problem was resolved.

So - if you buy an alternator, see if they have the ability to bench test it before you take it home and put it in. Good luck!