Poor guy!
Just a thought... did they run a lymes test? It's a fairly simple cheap test. Jackson came up limping randomly one day, and after x-rays proved nothing wrong, we got a positive lymes test, which affects joints, etc. He was better after a few days of pain meds and about 3 weeks of antibiotics.
Also, I don't know why they'd need to put him under for x-rays. Most don't. Jackson just went back and was back within 10 minutes or less.
Sounds like you need a new vet IMO.