This could be an odd "ground loop" so to speak, a problem with the regulator, alternator brushes, or problems in the bulkhead connector / ammeter / light switch circuit.
First thing to do is try and more define the problem. Get the thing warmed up and normalized, and try and determine if there's a "favorite" condition in which this occurs, IE dead slow idle, fast idle, low cruise RPM, etc.
Monitor battery voltage and record it. Turn lights off and see if voltage stabilizes. Now try and duplicate the load which the lights put on the system, that is, recheck battery voltage with radio / stereo, heater, etc powered on.
Best, if you can, to "drum up" a carbon pile tester, then you can vary the load.
Read this MAD article:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
Watch "whut it does" and determine if it's random or "rhythmic" If it's random, probably a loose connection. If it has a rhythm, probably some sort of loop
EXAMPLE.......loose connection in some circuit causes charging voltage to sag, regulator tries to compensate and at some point the connection "makes" because of the rising voltage, now the regulator must drop back down, over...and over.........
Check regulator grounding and check regulator supply voltage for "harness voltage drop."
To do that, run engine, warm, all accessories off, and repeat with lights, heater, etc on.
Check the ground by stabbing one meter probe into the top of the NEGative battery post. The post, not the cable clamp. Stab the remaining probe into the mounting flange of the voltage regulator. You want a very very low voltage, the lower the better, and zero is perfect. Over .2V (2/10 of one volt) means you have a grounding problem. Regulator MUST be at same voltage as battery NEG
Now check the harness. Turn off engine, turn key back to "run." Stab one probe of your meter into the top of the POSitive battery post. Stick the other probe into the connector for the IGN terminal of the VR. Again you want to see a very low reading. Over .3V or so (3/10) is too much, and indicates a voltage drop from the battery, through the various connectors and ignition switch, to the regulator.