Well I lucked out - it's fixed.
I removed the supply side and watched while I drained the tank, and it ran in a nice steady stream all the way to the end. (At this point I decided that it probably wasn't foreign object in tank).
Backtracking, I didn't have this issue till I changed the fuel filter, so next step was to open filter, and blow out the inlet side of the filter adapter housing. This seemed to be obstructed, so I removed the adapter housing from the tractor, and on the bench, It was obviious the the fuel cutoff was closed, even though the level indicated open. DIsassembled it shutoff petcock, and foud the problem was that it had been assembled wrong! There is the cast lever which has some nubson the inner side. They are supposed to engage a slot on the nylon valve core. For some reason there was a spring inserted between the valve core and the switch lever which was preventing it form engaging completely. I didn;t have the shop drawing, but it seemed more logical that the spring was supposed to be on the other side, maintaining pressure against the lever, so I reassembled it that way.
Operated for 1.5 hrs at
PTO speeds (shredding the field) and it ran perfect.
I'd love to know when the spring got moved, but I guess that'll be a mystery forever.
THANKS to all who suggested how to continue diagnosis. I really appreciate the expertise.