Thanks guys for the tips.
More careful inspection was the ticket. I thought I did a good job of keeping the terminals clean as the miles rolled by. What I hadn't done was peel back the shrink tubing that covers the crimp on the main cables. Positive side was caked full of corrosion. I must have wiggled them enough on my first voltage checks to re-establish contact. This was never a problem in pre-historic times with cast lead battery connectors.
We're clean and running for now.
