My guess is a fuel delivery problem, its leaning or flooding due to the attitude of the airplane in the air. Try running the airplane on the ground while another helps you lift the plane to vertical it should not really change how it is running if you are getting good fuel flow. If it does change or causes problems I would replace my fuel lines and check tank height in relation to the spray bar in the carb.

Just a gut feeling but it really sounds like a fuel issue.