Bobby at Cactus suggested this to me and it solved all my problems with said engine. Remove the 4 screws holding the carb to the engine. There is a phenolic resin block that separates the carburetor from the engine block. It tends to warp from the heat. After removing it, check it for flatness with a straight edge. Mine was warped about 1/16 inch. Put some sandpaper on a glass surface and rub the block until it is flat again. Replace it back on the engine. Note there is a rubber gasket that fits into a recess in the block. Make sure it is fitted within the recess before you tighten it back down.
My symptoms were rough running after a few minutes in the air and quitting in the air at low engine rpm. Mine would generally start OK after it had cooled off. Still I would check the phenolic block as I suggest. If you already have the cowl off, it only takes a few minutes to check this.