I just had this same issue on the Freighter going from KONT to KPHX at 31000 ft, the plane just started falling flat, the airspeed was okay then started overspeeding but the plane was still flat, no control of aircraft whatsoever.
Great job on the A300! Could you adjust the empty weight of the freighter so it is more accurate to the real freighter? As it is the empty weight is set the same as the Passenger version, which limits it's payload capacity. The Freighter should be able to carry a payload of close to 120,000 lbs. Thanks!