On your electrical woes, and possible causes of start box issues: grounding is always a good thing to verify, and you did that so next I wonder if you checked the integrity of the positive battery wiring on the backside of the battery box? There is a post that is mounted through the battery box for the +24V line where you connect the batteries on the wall of the battery compartment, that post has a similar side on the outside of the box. If you haven't, crawl under there and make sure that side of the connections is tight and not able to contact the starter casing. The starter is very close to that post, and in my opinion that is a weak point of the design. I had voltage fluctuations I couldn't figure out, but then realized no matter how tight the battery terminal clamp is, there is another connection on the outside of the battery box, and mine was loose...
BDGR