We had a cat come in a year or so ago, which had similar symptoms as you describe. We tried all the usual treatments and nothing seemed to really help the poor kitty. Eventually a diagnosis of Herpes (aka. Rhinotracheitis) was given and the cat put on L-lysine and seems to be alot better. From what I understand it may never be totally normal again, but at least it's a big improvement.

I dunno if this is what your cat has, however if it's an unvaccinated barn cat from unvaccinated parents I suppose it's possible that your cat has the same problem.