Drew..
My 12v source is a 12v battery charger/booster that I have tested up to 200 amps setting. all connections on positive side have been cleaned. bad ground is something I haven't explored. Good idea. I can possibly use jumper cables from starter mounting flange to the battery negative. Will do that now.
I did actually replace the starter motor with new. I really have the same issue I had prior. Interestingly there were FOUR of the plastic "bendix" type drives inside the starter mounting hole that had been bouncing around in there for a while. Apparently previous owners had starter issues as well. It really was starting and running well prior to this last time I let it sit too long.
