When the last time, I fly the phantom, I noticed sometimes the copter rotates slightly without my control. The GPS was good with no red light flashes (that means it had enough satellites). I've experienced some slight sliding previously, but never noticed that rotating. I plugged the copter in to PC software and checked, whether it needs a compass calibration or not. But it says, doesn't need a compass calibration. - Could anyone please tell me, what may be the problem?