Bouncing Voltmeter

[Mustang Don]

I have a 1972 Mach 1 and at higher rpm's above idle my voltmeter bounces from 11 to 16 volts. If I turn on the headlights the bouncing stops and the meter reads almost consistently
12 volts. I redid all my ground wires and cleaned all metal parts for a good ground. What am I missing?

 

[2Blue2]

Wiring and grounds are the first guess, If wiring is 100% then
Could be as simple as the belt slipping
Odds are in favor of regulator having greater resistance in it between charge and discharge when not under load, for what ever reason(old style has points inside)
Alternator going bad is next

Hard to diagnose electrical problems over the interweb
Keep us posted this problem comes up a lot.

 

[Mustang Don]

I put on a new alternator belt and changed the voltage regulator to a solid state regulator. I have no clue where to go from here.