I'm working on Octocopter with Pixhawk 2.1 + Here +.
The problem is the aircraft stop suddenly while flying constant speed with constant elevator or aileron in loiter mode.
It works great in stabilize or position hold mode.
So I've checked all 6 other aircraft with same configuration and they all have similar symptom.
The figure above explains what's going on.
I tried to upload the flight log file but I was not able to upload it here.
I can also send you the video clip explaining the behavior if you need.
Thank you very much for taking time.