I'd vote for the engine being tuned too lean. If it's exactly right on the ground with a full tank then it will be too lean in the air with a low tank. Tune your high speed needle to just off peak with the nose pointed up. That will be the leanest it'll ever run in the air and will put you a little rich sitting level. Then tune the air bleed for good idle and transition. If it won't idle for you for a minute on the ground then either your tuning is off or you have the idle set too low.