The 9C simply can't process the commands fast enough if you're using fast servos with a lot of throw. I could make my Funtana do it on the ground all the time. It didn't effect me in the air as much because you arent moving the elevator up and down all that fast and in the areas that you are, like hovering, you arent moving it far enough.

The 7C 2.4 shouldn't do that but I haven't played with it personally.