ALT gauge

"Something I forgot." Another thing to try, this will eliminate the charge line as a problem

Go to Lowes or other outfit that will cut wire and buy some large gauge wire, at least no10 and preferably no8. Get some eyelets on it even if you have to use the "house wiring" ones that clamp on. Hook it to the alternator stud and jumper over to the starter relay stud. If the VOLTAGE settles down, it's probably a bad bulkhead connector problem, or ammeter as mentioned.

When you do this, your ammeter will no longer be functional. You should, however be able to tell with a change in the voltmeter readings