JT
As Rotormatic and others have hinted on;
May I make the suggestion that you go all the way from the (power in) source, either way negative or positive, to the (power drain) source on both wires. Especially your earthing wires.
Truly, I have no idea just how many starter motors, and baterries, regardless of the type of machinery that were changed simply beacause the earth wire was not up to it.
I have Absolutely no idea how many helicopter gauges have been changed in my proximity, and many at cost to myself, simply because the same damm earthing circuit was not up to it.
Right now I have a problem with the engine instruments on two of our R22's in that regard. Every experienced driver will have a similar story to relate.
Many times I have seen that a starter motor will actuate because of the sudden high impulse it will crack an otherwise tenuous connection. I.E. one that will not allow a small current such as a battey recharge across the same connection. Result? flat battery.
I reckon that if you have fluctuating, RPM from either, of your ships or external power source, then you have a power transfer problem.
Voltage may well be really good in the unloaded state but current, as water, will only flow when you have a proper tap opening. That is proper cable cross section, and proper connection integrity, every connection and every cable, of which in your situation there are several.
cheers tet