Voltage steady when drawing on battery, but jumpy when on Alt..why?

I can't draw from my own experience here because I don't have it. Every time I've upgraded to a isolated field alternator I have added the 1 wire needed to isolate the field, along with that more complex voltage regulator. The higher educated engineers did that for a reason ( better control the field current / charging system operation ). I don't pretend to be smarter than those engineers.
So... 10 thousand posts on this interweb state, "just ground 1 field terminal ( retains the closed field circuitry ) and it will work just fine". Does that apply to all of the millions of various examples? Maybe. I'll never know if it applied to mine.
There is documented proof that the corporate bean counters omitted 2 of #158 bulbs from a certain Dart instrument panel for a time, to lower cost. That tells me they would not have added a 2nd approx' 5 ft. length of wire from the alternators field back to the regulator if it wasn't necessary.
In the end, Just sharing what little I do know. Good luck with it.