Over the last 3 or 4 flights I have the following regular occurance with a calibrated gimbal. After a couple of minutes into a flight (the time varies) with the gimbal pointing straight ahead, it suddenly drops (in pitch) by about 10 degrees so that it points slightly nose down. I am not doing any particular movement when this happens and so cannot associate the problem with any particular cause. If I use the scroll wheel to drive it back up, it goes back up and seems to stay there, unless I later move it intentionally. Any clues ? Thanks.