For your starting issue, I would start off with cleaning the grounds and looking for any corroded connections.
Even if the grounds look "tight" that does not mean they are good.
Take them apart, sand/grind them clean and apply dielectric grease when reassembling them.
I have seen some crazy issues resolved by "re-doing" grounds and cleaning suspect connections.
Hell, on occasion, I have even added more grounds "just in case".
Edit: True story about grounds
I went to out and started my truck. Fine, no problem.
Then I actuated the plow and it died. ......Huh?????
It would not restart (crank-no start) and had set a PCM code that would not reset. After hours of Googling it, Everyone on the internet claimed that my PCM was bad, the sky was falling, locusts were gonna eat up the fields and my truck was FUBAR. (oh and the whole "you should have bought a Ford" BS too...LOL)
Finally, out of sheer desperation, I decided to disconnect the batteries and let it sit. (trying to reset PCM code).
It worked. Truck started.......However, being the dumbass I am, I tried the plow again and BAM. It died again and reset that code.
I threw it on the hoist and I found
one bonding strap broken. I thought there is no way in hell this little bonding strap was causing all this.
I found a piece if wire, ground clean 2 areas and connected the ends.
Problem solved.
I now have 4 "additional" grounds.
I not saying that this is the OP's problem, but it is possible.
Just sayin....