bouncing volt and ammeter

This almost has to be one of several things no particular order

1....What is commonly called a "ground loop" although that is not always the actual situation

Check voltage between battery actual negative post and the VR mounting frame. Do this with engine warm and running fast as in "low to medium cruise" RPM. Check first with all accessories off, and again with lights, heater, etc powered. You are hoping for a very low reading, the lower the better, and zero is perfect

The VR MUST be grounded and at the same voltage as battery NEG. Remove VR, scrape around bolt holes and rear of VR flange, and remount with star lock washers.

2....With key in "run" and with engine stopped. Check voltage between VR IGN terminal (or as close as you can get) and battery POS terminal. Again, you are hoping for a low reading. More than 3/10V you need to improve this

3....Might be bad VR

4...In rare cases might be battery. Best is to try a known good battery and see if the symptoms change

5....This could be bad connections/ terminals/ connectors in the charge wire path from alternator to battery. Make up a temporary jumper out of some BIG wire, at least no10, or maybe a couple of no12 in parallel.

6....If nothing else works run a made up temporary jumper direct from the battery (or starter relay junction stud) to the VR IGN terminal. Don't forget to unplug when shutting down

7.....Further ramblings. I like to add an additional ground cable from engine to body. Look at the front passenger side of a V8 head, these same bolt holes are on the rear of the driver's side head. Buy a 1 ft or 18" "starter" (eyelet to eyelet) cable and bolt to the head and a good ground on the firewall, either a through--bolt, or to the master cylinder mounting studs.