It sounds like you just have too much throw and maybe along with an aft CG. Try backing out some throw just until it wont do it. This is a fairly typical high speed stall caused by exceeding the wings useful AOA for that particular airspeed. For 3D you would actually use more throw but at a much lower airspeed along with being in a stalled condition anyways. Usually for non 3D aerobatics one would not need more then 20 degrees of deflection.