1964 Other Pontiac Models headlights dim when directional si

I first noticed the headlights dimming with the directional signal when it flashes. But then I noticed that the headlights dim when any electrical thing is energized: heater fan, brake lights, wiper...etc. When the car is running the voltage at the battery is 12.8 when the car is not running it is 12 volts. The voltage at the alternator is 15 volts. The car has a mechancal voltage regulator and I am starting to investigate if it is putting out enough voltage.