The small constant voltage regulator is only for the non-ammeter gauges. Don't mess with that one unless the other gauges are not reading correctly.
The ammeter bounce is probably because of the alternator regulator. It is about 2 inches square and is found near the front of the engine compartment often near the battery or sometimes on the other side with three or four wires coming from it. The original regulators are old technology, get rusty, and the interior points don't open and close correctly. Get a new solid state one from the local parts store. They are relatively inexpensive and will probably fix the problem.
Ken
