Autotune makes quad tumble

Dead cat frame, APM 2.6 Firm 3.1.1, external GPS and Compass, 3d bat mon and sonar
I have managed to tune the APM so I can get a rock solid Alt Hold at any height, Loiter is also perfect. I engaged Alt Hold at 1m and then switched in Autotune. The program started with the left twitch then the right twitch then the quad just tumbled on to the ground. It did this twice then I quit trying to tune and flew about for 10 mins checking loiter, alt hold, stab and RTL. All worked well. I need to know why the autotune is causing the quad to quit.

do you have your sonar on or off during autotune?

My Quad did the same thing, stabilize work fine but as soon as I engage autotune it drop like a brick, some people in my groups suggest ESC/Motor lost sync