Wow...he has FABULOUS bloodlines!
I too, would do some serious checking, to rule out anything that would cause such intermittent lameness. Navicular in a mild form could present that way; work brings on the symptoms, then the rest helps them subside a bit.
He looks like a really nice horse; although, I have to ask, what, if he is such a calm horse, do you have to use the chain over the nose for? The chain overtime, can damage alot of nerves in his face, so I would becareful how you use it...Perhaps, get a Rope Halter with some extra knots instead; it will still have some 'bite' but will be gentler and safer in the long run.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."