Stock voltage gauge acting up.

Post back with what you come up with. By the way, what year/ model we working with?

Probably the very first thing to do is pull the brushes and inspect or replace them

Then, if you have the older 69/ earlier system, unhook the field connector from the alternator and use a clip lead between the alternator field connector and the battery. Start the engine, slowly bring up RPM, and see if the gauge is steady. Play with lights/ heater, etc to add load to the system.

If this "stabilizes" the gauge, then the problem may have been the brushes, try hooking the regulator back up

If the gauge remains unsteady, you probably have bad connections in the charge wire and your no1 suspect is a bad connection int he bulkhead connector

IF you hav ea 70/ later (isolated field) system, in the test above, you must disconnect the file wires (2) ground ONE, and hook the other to the battery as above.

This unsteady charging could ALSO be a major problem in the alternator, IE vibration could have ruined the insulation on the stator winding. Try running the car, with the field hooked up as above, in the dark. Look for a miniature lightning show in the stator windings.

Please read this article on bulkhead connectors and their problems:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml