Have you clean those electrical "buss bars" and connections for those circuit breakers, on the wall between the cab and the engine (inside engine compartment)?

{I have one circuit breaker, that operates part of the lights, that I've replaced 3 times, in two years, on a JD 6620SH.} I did clean my "buss bars", too, and all those little wire connectors, mostly one post at a time (so nothing got mixed up)



Now, I'm just thinking out loud here, BUT I wonder if it would help to clean your starter cable connections, too = I believe the alternator wiring, goes over to the starter solenoid, then the "juice" goes down the big cable, to the battery + post. 
I'm also wondering if maybe you should add an additional 12 ga. wire to the alternator's battery post(+) and run it over to the starter solenoid's post (+) because you have a larger alternator on it compared to factory amp rating.  {maybe the factory wiring, size wise, is hurting you a little} or maybe some corrosion has got into factory wiring, over the years, and is hurting you on your full Amp capacity.


Where do you have your GPS power attached? Directly at batteries?