I am sorry to hear of the trouble you are having with your cat. Conditions like this become very frustrating and hard on the nerves to say the least.
I've been thinking about your problems today and after seeing Barkingdogs reply to your comments of earlier. I think his suggestion of using a chiropractor might be worth considering, considering what appears to be neck or spine problems. Something else I think might be worth looking into would be homeopathics or naturopathic medicine to see if a practitioner could shed some light on the problem. The way you speak it appears as if you have a vet that you can work with, and has animals well-being at heart. If you think he is open to alternatives I would be running it by him and getting his ideas, and possibly he could give you some referrals, and possibly suggest a chiropractor.
I had similar problems years ago where the vet had hit a brick wall and didn't know what to do, or where to go. A friend suggested alternatives, and we found a vet who had practiced regular medicine for 12 years, and decided to expand her knowledge into homeopathic medicine. She was able to put her finger on the problem, which proved to be a comedy of errors, vaccinations, spaying, long distance travel with strange animals and of course a new home, not to mention and overprescribing of antibiotics over extended period of time. This raised supreme havoc with her metabolism. The damage done left her with no alternative but to treat with homeopathics and the cat is still with us today.
I think your vet would more than likely appreciate knowing what the problem is that he is unable to solve. So have a chat and see what the vet suggests. We would be anxious to hear the outcome.
Animals are such agreeable Friends.
They ask no Questions. They pass no Criticisms.