Alternator gauge very erratic.

Step one: Bypass the gauge and the bulkhead. See search engine on this very site for instructions. failure to do so invites melted wiring and vehicle fires.

Step two: replace (as mentioned) the main voltage regulator. If yours is still electromechanical, it will behave as described. Conversion to solid state is cheap, easy, and well worth it. Again, "search".

Step three: replace instrument module voltage regulator (OE was electromechanical, solid state is a DIY thing, or purchase from vendors found on this site.

For my car, it took all three to get stable lighting and the IM regulator was the major culprit.