The same thing happen to my 6 year old cat, she was fine and healthy, they did test, xrays you named, the vet told the same thing that yours said, it was a neurological condition like a stroke and probably genetic, she die couple days after that, I took her to the vet to put her down but she died few minutes before. There was nothing you could do somentime things like this happens
, so very sorry, hugs.