This actually reminded me of my horse. He was only lame in the fronts and only at a trot.
At first I thought it was just his arthritis acting up and the crappy arena we had. But then it would just happen randomly without any work and in the pasture. So I had him checked out by the vet.
We did a block and some x-rays. Turns out he has high pastern ring bone. Which when he trots, the hard pounding of his feet, cause him pain. When he would show up lame in the pasture, it was either because he stepped wrong or was screwing around with the other horses.
I would have him checked out for that.
ringbone is typically caused by too much training at an early age or bad shoe jobs(in my horses cause both).
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."