Don't know but have you check the tracking? I just fire up mine with weight on the skids and watched the head and the thought I had is, " is everything smooth or does something jump when you pull back on the stick? Also how high are you off of the ground when you feel it suddenly drop? Could it be ground effect and reaction to it going tail in? I've noticed that when I come to a stop from FF that I have a tendencey to over stop and go backward and drop a bit. If you have a way of strapping your Blade down, not just holding it, and getting a video of the head movement and servos someone will get this