Stock voltage gauge acting up.

Do you really have a "voltage gauge" (voltmeter) or the factory ammeter?

Very first thing is to "rig" your multimeter to a good solid battery connection and go for a test ride to see what the voltage is REALLY doing.

Otherwise, it could be a number of things, including but not limited to:

The meter itself could be bad

The alternator or at least brushes bad

A bad electrical connection anywhere from the alternator through the bulkhead (twice) at the ammeter if still in use, and to the battery

Other problems in the alternator

Bad regulator, bad connections in the regulator

Under what conditions exactly does this happen, find out FOR CERTAIN are we referring to a voltmeter or ammeter, and monitor the voltage as suggested above with a multimeter

Is it affected by system load, IE worse/ better when battery is charging after a start, or with lights/ heater/ other on, etc?