john-l
You seem to be going around in circles here. The problem may not be the carb. I had a similar problem after about a year. The XT has a side stand safety interlock switch which will ground out the ignition if you try to ride with the side stand down. Sometimes this switch can become dirty as it is located on the side stand. Consequently it sometimes sends false signals to the ignition. At low rpm there is not alot of vibration, however at high rpm with alot of vibration the switch could be making intermittent contact causing the ignition to cut in and out. You cannot "legaly" bypass this switch, so I recommend replacing it and see what happens. It solved the problem for me and a friend with the same problem.
If you end up going back to the carbs I would advise checking the jetting against the altitude you're riding in.
The pilot fuel screw is mainly for idle. About 3 turns out from fully seated is a good ballpark setting. Anything above idle is controlled by the jets and needle setting.Just a few suggestions.Good luck.

mm,
I have just been driven mad by seemingly the same problem.
I have replaced the mains switch and ignition coil, to get slightly better results.
WIll try everything you have all suggested before rushing off to a mechanic for an expensive solution. (i don't really have the time anyway as I leave on SUnday)

Geoff