If you're going to have him checked out again, I'd have them check his hip. My horse's hip was out for a long, long time, and they kept fixing his shoulder, because that's where it would show up! He bucked through riding whenever his hip was out, and we had to have it done three times before it stayed in place-knock on wood. >.<
I don't know if it is necessarily a pain thing, either. Some horses are fresh, and don't always calm down, so maybe it's just because of that?
How old is your horse?
"Yes. Like 'Wendy'...With a T."