More than one thing can be happeneing here. First of all the SS deosnt have any down thrust built into the motor mount. This can cause the plane to climb while the prop is doing it's job. I think I remember reading somewhere once that the recommended CG is wrong, but I cant find it. Google Slow Stick and look around.

Question though. When you cut the throttle, what does the plane do, attitude wise? Fly level, dive?