My friend's horse did almost the same thing. He would be fine then suddenly acutely lame, three legged hopping. Vet & farrier found nothing. A chiropractor found something out in his neck & withers. One adjustment fixed him.
Maybe try another chiro? X-rays would be good too, maybe of the neck, as in his picture he appears to have odd muscling, a kind of circle on the lower side about midway between his throat & shoulder. It kind of covers where you would normally see the neck groove.