Problem is solved! I want to sincerely thank all of you on the forum for ideas, thoughts and suggestions. It seems that first impressions can sometimes be right on the money. My first impression was that there was a breakdown in current flow with high demand. I thought maybe battery or cables and swapped batteries. That didn't change symptoms. I bypassed all safety switches no change. Pulled ground cable and cleaned connections, no change. Pulled power cable, removed plastic boot and noticed that something didn't look right. I hadn't noticed anything when swapping batteries, but hadn't removed the boot. What I found was that the cable had badly corroded at the battery clamp and the boot was the only thing keeping the cable and clamp in contact with each other. Somehow, pulling the cable when swapping batteries didn't result in complete failure of the cable and symptoms were exactly the same.