When you adjust the model's carburetor with the nose pointing upwards, you are anticipating two problems. The first is the reduced pressure head of the fuel in the tank as the fuel burns off over time. The second is the engine "unloading" when airborne. Actually, the second issue is not a problem, because when a two-stroke unloads, it actually needs less fuel, not more. But this is an R/C myth, so we shall humor it.

What is happening with your engine's idle and transition is that you have NOT anticipated the leaning effect of burning off fuel as the flight progresses when it comes to the low speed adjustment.

If you adjust the low speed needle to its perfect idle and transition mixture when the model's tank is full and sitting on the ground, it will become leaner as the flight progresses. Richen up the low speed a bit and see if it improves things.

One must keep in mind that we are dealing with averages when we adjust our suction fed glow engines. We never want the engine to run too lean, even if it means that the throttle is a little sluggish in the beginning of the flight. Good flying.