alternator gauge drops and reads low when driving

Check your wiring behind the gauge cluster. I had a problem with a 69 Valiant blowing alternators and regulators every six months. The gauge would jump every once in a while. Turned out to be a melted wire. The main power wire from the alternator to the amp gauge had gotten so hot that it melted the insulation off and was sometimes grounding against the metal dash frame. After replacing all of the bad wires and new alt and regulator, I never had to replace them again.