It's a common problem that I have never found an answer for.. Drives me crazy. I calibrate on perfectly level surface (using piece of plywood and a level that I bring with me). I go on up and as I yaw, the horizon begins to tilt. It seems to be worse on windy days.
I think what's going on is that because of the wind the Inspire itself has to tilt into the wind to maintain position. And the cameras horizontal plane is in relation to the inspire, NOT the earth.
I could be wrong buts it's the only thing I can think of as it is definatly worse on windy days.
I really need to try to use horizontal camera control instead of yawing the Inspire next time I'm out just to see if it still does it.