Is there a veterinary school near you? Teaching hospitals associated with veterinary schools see a lot of hard-to-diagnose cases. Because they see so many out-of-the-ordinary patients, they can often diagnose things that regular vets don't ever see and therefore don't recognize.
The down side is that they can be fairly pricey, but if you get an answer and a treatment, you won't be spending any more money on fruitless vet visits.
We had a dog that was sick for a year before we tried the VTH at the Univ of MN. They diagnosed inflammatory bowel disease, got him on a course of treatment that knocked the disease back far enough that we could treat it through diet. Had we continued going to general practice vets, Evan wouldn't have lasted another month. So we felt it was well worth the money we spent there.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."