Hmm. Have you considered consulting with a holistic vet? Many will do telephone/ email consults and be just as effective at treating your pet. It seems to me since this problem isn't going away, there is an underlying health issue and immune system suppression.
Here is the website for the American Holistic Veterinary Medical Association. They have a referral section to help you find someone who is somewhat local.
AHVMA - Home
If this is the 3rd time you are doing the treatment, I would be considering alternatives. Traditional vets have very little at their disposal as far as treating these issues...Thats why you have done the same course of drugs 3 times!
A holistic vet will look at the whole pet, his medical history, what he eats and more and help determine a course of action to not only heal, but strengthen immunity and prevent future episodes.