Hi, Hope someone can shed some light here in whats causing this. OK I have setup a flight plan through Simbrief and exported it into the aircraft. I can see the flight plan on the ND. Take off climb and the Aircraft settles down at the desired Altitude and speed and on line with the plan. Decreased my throttles back. Then all of sudden Aircraft gores into a steep climb turns off AP. And all hell is let loose. Question : Whats causes this?

Hi @Timm Turner
Please ensure all your AI assists are disabled (Assistance Options).

The conditions that could trigger an AP disconnect include input on your stick (elevator or aileron axis) or also on the rudder (rudder axis). Please check your bindings as well for any conflicts/double binding or "noisy" axis inputs that could be causing this. 
Thanks!