ORIGINAL: Albatross
From idle goose, stumbles then finally reaches 6300, 6200.
Throttle back. let the engine idling, wait some time (no sure how ), goose then the transition is just fine this time.
Taxing, then open the throttle, exactly when is gaining altitude or before the engine stumbles.
As mentioned earlier make sure the engine is warmed up, especially before changing any needle settings.
Does it ever hesitate/stumble in flight when you go from an extended low throttle setting to high throttle setting?