I fly an OS .55 AX on a ModelTech SU-31 profile. The nature of the plane is that you spend a lot of time in a "nose up" attitude or high alpha if you prefer. This means your fuel is being drawn straight up (fighting gravity). This can cause a lean run if the engine is not getting enough fuel. Tank presure is a must to feed the need in these situations as well as a good mixure setting. Ensure you tune your engine so that it runs reliably in the high alpha atitude by holding the plane upright for 30 seconds or so while running at less than full throttle, about what you need to hover (less tank pressure). Good luck.