Strange behavior & crash


I had a flight with a zmr250, pixhawk lite, ardupilot 3.3.3 ending with a crash. It was planned as a short mission to test the settings. Start and the first wp seemed normal, but then after each wp there was a strange stop of the copter, slowing down and loosing height - and coming slowly back to planned height. After a while (49,2k) I did switch to loiter and did try to fly back. At the end throttle showed kind of erratic behaviour - suddenly full throttle - and the copter crashed.
Does anyone have an idea why the copter did not fly properly?
Thank you!