Dang, I should have thought of that in my first post.
15 years ago I had a bad ground in a '73 Cougar that I figured out when cranking the car with the hood up. The air cleaner snorkel wiggled over against the inner fender and sparks flew. That was what I call a CLUE.
A couple years ago, I had the same problem with my motorhome. I even had the danged starter rebuilt before I figured it out. I finally started tracing it with a DVM and found I had several volts between the engine and chassis during cranking. The ground strap connection had corroded. Cleaned it up and no more troubles.
Why does my brain reboot every so often and I have to learn everything again?
Hope this fixes your problem,