Ok heres the symptoms... truck is a 1989 bronco II , but it is a general voltage question so that shouldnt matter.... I have replaced the battery and alternator on this truck recently, battery and alternator both test perfect.. Alternator has an external voltage regulator that is mounted on the back of the alternator that has not been replaced. My battery needle stays around 12 or 13 volts unless I am starting the car, putting up the windows, listening to loud music, or turn on the headlights, then the needle drops a bit, and you can see it jump a bit when under a load and when using turn signals, the ground cable, power cable, and all connection have been checked and rechecked, all are good... would this mean a bad voltage regulator? I have never dealt with a regulator before until now, just used to swap in a new high output alternator and go... but this didnt fix my problems this time.
please message me or email me with any suggestions or diagnoses's. also the driver and passenger windows are electric, and go up slower than a fresh terd rolling uphill on a hot day.

 

If you checked the connections for corrosion and the resistance of the cables, I would say the regulator

 

is the gauge grounded good?

it can be a gauge prob have you actually watched a voltmeter at the battery when you turn the lights on and all that to compare it to the gauge?