Not my airplane, and I don't know if he fixed it. I posted the same video on RCGroups, and this is the best response I got so far:
Quoted from cfircav8r
Definitely not a stall. I have seen that exact thing while instructing in full scale. you are coming in to land get hit with a small gust or just pitch up too fast and the plane balloons. The common, yet wrong, action is to release back pressure and the plane dives back down too quick to react to. The correct action is to maintain the stick position and even add a little throttle if it seems a little slow. This will slow the descent and allow you to stabilize the approach. The instant you relax that back pressure it will dive, when you maintain the pressure it will still start to drop the nose and start a rapid descent but it will be a more controlled descent and you will still have some airspeed to play with. If you are too near your stall speed you have to either go around or use throttle, not the elevator, to slow your descent.