Now that I removed the engine I found the root cause; A broken mount.
Now I understand that the drag was really "twisting" on the drive gear and chain.
I had just changed to a new chain so the effect of a broken mount and the engine "twisting" at rpm makes sense now.
I asked why did it tend to drag when I had come to speed of about 27 mph or so.
Why did it sometimes not drag or bind up like something applying a brake when I ran at lower speeds such as 19 or 20 mph..
Now it makes sense to me. With the faster speed and greater pull it was twisting the engine and the drive gear was binding on the chain.
That is why the idler was wearing in a funny way.. That is why the drive bearing stands to be replaced for good measure if nothing else.
Here is a picture of the "Muffler clamp" broken in two, This is the root cause of failure as far as I can tell.. A sneaky failure since it stayed in place and looked essentially "whole." Not untill I removed the motor did it become obvious.
My idea of a beefier fix is a square steel tube rather than a stamped part which that "muffler clamp" is. The bends where the press bends a flat piece of metal into that shape are already stressed so adding the stress of an engine means it's just a mater of time before a hairline crack starts and eventually will break off part of that piece.
Well this adds a delay as I consider what to do to fabricate a replacement. The actual piece is a nice design but I have steel tube to work with. The tube gripping curve area will be smaller however I might get the broken pieces ground down and welded on the tube thus giving it a larger "grip" area..
I have to think about it.. I am still working 7 days a week.. Kinda tired.. I do have a ride for the week but I am ready to get back on and ride...
Addendum : I ride 24 miles a day 7 days a week so if it is going to happen I will see it...
I have another 40 days to go so at 24 miles a day so 960 miles to go means I will have a goodly experience of rough and ready riding.. All the modes and fixes I do now are related to honest, rut running and bump bashing daily grind of country road distances.. Real time and real wear on the components we buy and use every day.