The snapping is typical for a plane with 3d rates. You basically need to fly normal LARGE loops (with very little elevator) or do full "up" loops at low throttle (a waterfall). with full up or full down elevator, the plane will not fly on the wing. So keep your plane up high while you get used to flying 3d.

You should be running a lot of exponential on the elevator and ailerons. Or use dual rates - one for 3d and one for regular flying. I suspect your plane setup is fine. A rearward CG can make for tricky landings, so don't let the plane get too slow. You might have to push some down elevator on final approach to keep the nose down and keep it flying.

If you have a 3d flyer you trust, have him check it out.