Low charge when hot.

So I went on a cruise last night and it was a lot of stop and go traffic. Car was running about 200, no biggie there but I was noticing coming to the stops that it was not charging or barely charging. The voltmeter usually is about 14 but this was about 13 or just under, sometimes 12 and I would put it in neutral to help it a little and get it above 12.5.

What would cause this? It has only happened when I'm driving it for a while in stop and go traffic. Long highway drives are fine and general running around is fine also.

Car is a 71 Dart with 318.