ALT gauge

Could be several reasons. You mean it bounces some, or does it wildly peg, or exactly what?

69? What you running for charging system, IE orginal type Mopar, or converted to something else?

I tend to divide the charging system into sections.......

Is it field wiring?

Is it output (charging line) wiring?

Is it bad alternator?

Is it bad regulator?

This problem can be tricky. "Let's say" it's a bad connection in the charging line, most likely at the bulkhead connector, but could be at the ammeter itself, or (rare) the "welded splice"

Review this article:

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

and the diagram on that page, which gives you a simplified view of main power.

"What I'd do."

Might be intermittent / worn brushes, loose connections in the output line, or loose / bad stator / diode connections in the alternator.

First thing I'd try is DISconnect the regulator. You can just pull the green wire off at the alternator. Connect an alligator clip lead at that alternator terminal, and jumper it over to the starter relay "big stud." (battery)

Now run the engine, slowly bring up RPM, and see if the problem is still there.

If so, it's now either the alternator itself, or the output wiring.

To eliminate output wiring is a "bit" tricky.

Start by running the engine and monitoring battery voltage with the clip lead in place, bring voltage up to "about" 15---15.5 and try to maintain that.

Next, move your voltmeter hot lead over to the alternator output stud. See if the voltage seems "way more unstable" at that point.

If so, you likely have a loose connection in that line.

If the voltage is fairly stable, it is more likely a problem in the alternator.

FRANKLY I just HATE to recommend "throwing money" at a problem, but this is one of those times when it makes sense.

Consider investing in a new alternator. If you buy a 70/ later "square back" you can ground either field terminal and hook the thing up just like your old one.