You've mentioned a lot of things your vet has ruled out based on chemistry etc. What I'd like to hear is what he has found on clinical examination, and what the medical history of the cat has been. Have you ever dewormed, vaccines, feeding, past problems and other cats with problems, ectoparasites, mosquitoes: you really need to pressure your vet into saying what it might be, what his diagnostic theory is, not just let him run you up a large bill on machinery! Good luck, and there is always a reason for the problems you describe, pressure your vet into finding out!
