I have the same airplane, first, you need to make it tail heavy, move your cg at least 1/2 inch more back. also, you need to give full travel on your rudder and elevator. ailerons also need much more travel as well.
remember, this is a heavier plane for a 3d aircraft. the rudder is not large enough for proper flat spins, making a addition to the top of the rudder will really help. if you want real 3d, you need more surface on your rudder and elevator. also, take out as much weight as possible, light wheels, light batteries, every ounce makes a difference.
all of this will help, but will not be as good as a lighter plane, and you need a good power plant with a large diamater prop with around an average pitch.

GB