If at all possible, I’d go to a clinic for a full work up with a lameness specialist. I’ve seen weird behavior turn out to be caused by things you’d never, ever guess.
For example, top-flight A/A hunter that normally jumped with his knees to his chin–and so square you could balance a ruler on them–started leaving his right front back, subtly at first but gradually increasing. Farm vet came out and of course we started with the right front. X-rays showed only minimal changes to the “track jewelry” seen on the PPE 5 years previous and nothing more, so we injected. Horse still losing form and then stopped for the first time in its life.
Full work up at the clinic found a SUBSTANTIAL suspensory tear in the left hind. The horse never took a lame step, but if I’d not gone to the clinic and just kept trying this and that, I probably would’ve ended his career–if not his life. By the time we got the diagnostics, it was almost fully detached. Instead, he came back and was Zone Champion for me again, then I leased him to kids (both were also Zone Champions in the pre-children’s) and today he’s 27 years old and roaring around his retirement field.
Another one I knew suddenly started stopping several feet away from the jump and spinning violently. Turned out to be a pulled pectoral.
A friend’s horse started stopping and spooking and no amount of pro riding could fix it. Sinus infection requiring surgery. Went back to being a complete packer.
How about the one that started bucking after jumps that turned out to have somehow broken her withers? She only did it when the rider was even just a touch ahead of the motion.
Or the one that would periodically buck and plunge on the landing side, usually about a week before he was due for a reset? Soft tissue injury inside a front foot not seen on x-ray but absolutely there on MRI.
ALL of these horses appeared completely sound, were lovely to flat and snuggle bunnies in the barn. Some horses are super-stoic “doers” who keep trying their best while also communicating something is wrong. Training on these horses would only have caused further damage, but they also all had ailments unlikely to be found by a regular farm vet going by trial and error or process of elimination.
