Originally Posted by
on the path
Tighten up the rear skewer.
You sir or madam are in my debt. If you are in SE Michigan I owe you a beer or 2. There are others with the same suggestion if I recall. Thanks to you all.
I feel foolish, dismissing these suggestions, but it finally clicked. Loose skewer, flexy rear triangle, especially under load. Duh! Just came back from a ride and the angels sang once again.