Not entirely sure of the whole situation as this isn't my horse, but we trust what the trainer says. Last month the vet was out for teeth and check-ups, and the trainer gives him frequent rides so she knows the horse. So far, he hasn't done it with her, but it's another story with others. He goes weeks, even months with no problem whatsoever, and then you'll get a ride where he seems out of his mind. I personally rode him a few weeks ago and didn't notice anything out of place, his ground manners are fantastic, but then again, what do I know?
Can you think of anything else other than pain that could be causing it? Thanks, I really appreciate it!