Phantom 2 with H3-3D gimbal had a few rough "landings" due to ring state vortex. Replaced gimbal, checked software update, did advanced IMU calibration* and controller calibration. Compass calibration is good also. (* I did get a warning that MC was too warm during IMU calibration but ignored it because it was at room temperature) Shell does not appear to be warped or damaged. What else would cause it to drift severely?