This can be cause by a number of things
ammeter got hot, loosened up, and has poor connection.
Poor connections ANYWHERE in the harness, including connectors, in-harness splice(s) the bulkhead connector, and the alternator itself, including brushes and a partly shorted stator.
In the dark, engine running, lift the hood. Turn on a heavy load, like the heater motor. Rev the engine up/ down and look for "lightning" in the alternator. If you can see sparks coming from the stator windings, they are shorting from vibration and have come loose, worn off the insulation.
Can also --depending on how the meter is acting-- a worn/ slipping fan belt
Might get an electronic parts house replacement & someone here had a black OE appearing unit for $25 to your door.
