Hm m m . ...I still think it's pain related, don't you? I mean do you get the feeling that he's doing it to get you off so he can go back to the barn?
When he is on the lunge line, is it at the canter? Does he canter bent to the outside? Does he seem really off balance on the lunge line?
It's possible that he has some sort of misalingment issue in his pelvis , spine or ribs that is activated only when moving a certain way. Taht's one idea.
When he wants to buck on the lunge line, do you let him? I mean, maybe he needs a good rodeo to just get the kinks out. He might feel better to really go to town once in awhile (unmounted, preferably).
YOu've also ruled out the teeth or bit being a source of pain?
What do you do when he bucks? Ignore? Smack him? Pull up his head? Drive him onward more vigourously?