Sadly, my nook has just started freezing a good bit and I always have airplane mode on. Doesn't seem like that's the answer after all.
Try defragging it and see if it does any better. You also might try removing everything off to your PC and delete everything on the nook and then move everything back. WHat I mean by deleting everything is the books behind the folders. Don't delete your folders just the stuff behind them.
Kevin