I am having control problem with my F550 as well. It wont hold height.
I notice in intelligent mode, it wont cut motor below 10% within 3sec without push power.
I follow everything in manual, calibration Radio, compass, Tx everything centre - trim and in aeroplane mode.
http://www.youtube.com/watch?v=btRTuj_7l4Q
This is show how unstable the vertical control - I try vertical gain using VR from 100% - 250% with no different.
Try turn off Gimbal and Lipo one time and have it work - which power cut off works as well - got 2 flights out of it.
Then I turn on Gimbal and Lipo and test..problem come back and now I turn off Gimbal and Lipo, problem still here.
Any ideas ?