I have a ST.90 that does the same thing, runs great on the ground, but dies at midrange in the air. Tried everything that has been posted so far with the same results. I'm going to try putting mine back on the engine stand and run another half gallon or so thruit, has about a gallon now. If that doesn't do, may try a Perry carb or just sell it super cheap on RCU,give up.and cry uncle.