Try checking the pull pull tension. Under high stress the airplane used to snap on me because on elevator had slightly more tension. I fixed this by just throwing the setup out and replacing with two elevator servos.

I do find it hard to believe a saito 180 spinning a 18x6w apc over 10k. what brand prop? I do know a 19x6 Bolly CF will spin at around 10200 on a Ys140dz.