Not charging

I took my 1964 Dart 270 out the other day. I noticed the battery gauge was pinned to full charge. This was not normal. I just thought it was bringing the battery up to full charge . After a while the gauge dropped but still not to it's normal position. It was at 3/4, usually just to the right of center. Later that night the gauge went to left of center and when I checked with a multi-meter it was not charging. I checked all the connections and the alternator leads for continuity and all were okay. I cleaned the points in the regulator too. It is still not charging and I don't want to jump to replacing anything without doing some troubleshooting first. What do you think has happened? Are there some more steps I can take to further diagnose the issue? Thank you for your assistance as always.