69 Dart - overcharging issues

I'll try and make this short and sweet.

Quick info: 1969 Dodge Dart, Car has single field alternator (round back) and mopar electronic ignition. (orange box, blue electronic voltage regulator)

Every now and then it started overcharging (bounced up to full C, 18V on volt gauge), I could rev the engine up quickly and it would correct itself.
Then it got worse and at any RPM above idle (1000+RPM) volt gauge would go to 18V, dash ammeter would peg to full C.

I had been having problems with the black charge wire at the bulkhead connector vibrating and causing NO charge issues. Someone`s previous wiring repairs to the terminals were less than desirable, so I ended up running a solid wire through the firewall to eliminate any issues there. (There had obviously been issues before someone else got to it too, due to the slightly melted plastic around the terminal in the bulkhead). Did that, unfortunately no change in over charging issue.

I then figured it was a faulty voltage regulator, so I replaced it (Electronic Mopar Performance one, PN:3690732). Scrapped the paint off around the mounting bolt and installed a ground wire from the mounting bolt directly to the battery -ive to ensure a good ground to the case of the regulator. Initially it looked like it solved the issue!

Multi-meter read battery at roughly 13V at idle, increased to 2000RPM and would get 14.6V at alternator & 14.3V at battery. Turn on loads, ammeter gauge would dip a bit then go back to where it was sitting before....

...but now there`s a new problem.

Went for a test drive, and the ammeter just started bouncing around like crazy! At idle it now sits at roughly 14.3V on my in car volt gauge, and revving up the engine causes my ammeter to dance around. It also, every now and then, jumping up to 18V and overcharging, but just for less than a second here and there when I rev the engine.

I just took the alternator off to go get it tested, just to rule it out as being the issue.

Looking for advice on where to start testing and looking next.

I`m really hoping it`s no wiring issues under the dash....