78 Trans Am charging problem

Hey All-
I have a 78 Trans Am that is having problems charging the battery at night with the headlights on. During the day it seems like it is OK, without the extra electrical load of the headlights.
What happens is the voltage gauge will read around 13 volts. Then while driving, it will fluctuate rapidly between 11 and 13 volts, with a corresponding brightening/dimming of the headlights. At times it will rise as high as 15 volts and stay therea while. Other times it will drop as low as 9 volts, and stay there a while, and the headlights get very dim. Then suddenly it will go back to 13 volts and the headlights get brighter again. Turning off the headlights and just having the parking lights on does not seem to help. This occurs at all speeds/engine rpms. It isn't happening only at idle or anything like that.
The battery and positive battery cable are brand new. The alternator (w/internal voltage regulator) is relatively new and the alternator belt is tight. It was loose but I tightened it, and the problem seemed to go away for a while, but now it is back.
Does anyone have ideas of what to check? I would rather not get stuck somewhere.
Thanks! -njot
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snipped-for-privacy@juno.com says...

It sounds like the voltage regulator in the alternator is bad. Unless you bought a completely new alternator, most of the time on rebuilt alternators they don't replace the regulator if it tested good at the time. The rebuilders usually media blast the living crap out of all the parts so they look clean, throw in some new bearings and brushes and call it "rebuilt."
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Have the alternator/vr bench tested.

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