Charging intermittent, sometimes it does sometimes not

Took my 70 duster for a drive today. For the first 5 miles or so it was charging normal per my aftermarket voltage guage. 14 volts, then when I looked back at the guage it went down to 12 volts and stayed there for another 5 or so miles, no matter the rpm, at idle at stop lights etc. Then it came back to life and charged at 14 volts again for another 5 or so miles then it went down to 12 again. When I got back home I checked the wires at the alternator to see if I had a loose connection, but they seemed okay. Do I need to look for a pinched wire or loose connection somewhere else or could this be a dying alternator or dying voltage regulator? Let me know where I should start. Thanks.