If him being "off" has anything to do with his feet, I'd imagine that he'd feel stiff or bobble in the trot. Does he do any of that?
It's more subtle than that. Always has been with him. As a rule he seems sound in the trot if not as animated as he is when he's 'on'. What I'm seeing is just a difference in enthusiasm. It's very vague, but instead of him being coiled and chomping at the bit to "GO" all the time as is his norm, he's walking on a loose rein like he's going somewhere he doesn't want to go.