Hi This is my first post, but I have been building and flying multicopters for around 7 years, from MK, through APM to now a hex with Naza and the Phantom 2. I had an odd flight last weekend and I'm puzzled what the problem was. This was the second flight of the session, starting with a full battery. I got a full lock with 10 satellites, green flashes, then waited around a minute after that, before take off. Throttle seemed fine, but I did notice a slight downward drift on centre stick. Not enough to worry about, but I wondered if the cold air was having an effect. This was a fairly cold (~6 deg C) but fine day. After a few minutes of flying, first in ATTI, then in GPS mode, the phantom started to slowly sink during forward flight. Once this had started, it would not respond to up throttle, but would respond to down. Once I had landed, I turned the battery off, waited 10 secs or so then turned it back on again. It flew without incident after that and was happy to hold altitude. It almost seemed as if the max height limit was coming into play much lower than it should have done, as if maybe the altimeter was way off calibration. Anybody else had this behaviour?