alternator gauge drops and reads low when driving

I have a 1973 Dodge Dart that seems to have a wiring issue. I only notice the issue when driving at night because everything on the dash dims down. I will be driving home from work and all of a sudden the alternator gauge will dump down and read on the first or second line. Funny enough there are no performance issues just a surge in my electrical circuit. I thought it may be the alternator so I had it tested and it's putting out around 14.9V at idle. I have had this problem occur when driving and it will run low for usually 10 minutes. Sometimes on the rare occasion it will last longer but I shrug it off because nothing else is going wrong at the time. Recently it has just been either doing the above or it will slowly drop down like the belt isn't tight then if I accelerate it comes right back up without any problems. I never had this issue with my '63 Dart or my '80 Aspen. Though I was the fool who blindly traded an almost finished 63 for a complete start over project. Any idea's on what could be going on?