I was wondering , is the bike getting hard to start?
chipped ,cracked worn reeds? (loss of signal strength)
If adjusting the air screw richer helped, could the tank vent or carb vents be restricted ?, allowing larger throttle openings enough vacuum to draw fuel and when throttle is chopped the sudden loss of signal strength shuts you down.
Not discounting the previous float level suggestions, as the symptoms would be just like a partially clogged fuel filter in a car. You could only run as fast as the fuel pump could keep the bowl full. Pulling in the clutch and trying to keep the old girl running may not provide the same "draw" as "coasting" pulling more fuel in to the carb.
There is a small filter on the inside of the tank attached to the fuel fitting that may
be giving you a problem as well as the small passages in the petcock itself.
Oy, I'm not very good 'Splain-en stuff
but maybe some of this might help find the buggar.
Tuck \o/