Erratic gauges

My 66 Dart has a problem where the gauges will run at normal position for a while then will climb up about a quarter of the way and stay there. At that point, if I turn the lights on and off quickly, or the heater/AC fan, they will drop back down to their normal running position again for a while.
Am I dealing with voltage regulator issues, or maybe that voltage limiter on the back of the instrument panel?

Appreciate your input.