Aww man Becky, that really stinks. It sounds like he has a stifle problem. It sounds like the joint is getting stuck, and he can't move it.
That's exactly what I'm wondering if it is. I've never seen a horse with a stifle problem, so not sure what to look for.
Could a vet tell, or is this going to be a guessing game? Would she have to see him when it happens to tell what is wrong?
Becky