This problem is something that is degrading from heat probably. Probably not the carb. Many good ideas have been mentioned. Sometimes a coil will do that, wires will do that, valve train will do that, heat riser, vapor lock as mentioned,.. if it's the carb at all, it's a piece of dirt floating in and out of somewhere. Keep thinking and testing ideas. It can even be a weak battery discharging too low, then magically rejuvenate upon "settling", I know sounds crazy. Watch a few UTube videos on BC rebuild to be sure you got all the right balls in the right hole and you didn't leave a stuck ball in there.
