My dog has the same problem, and my vet put him on some VERY expensive medicine, but after the antibiotics were out of his system they came right back. So, we took him to another Vet, and they said to try and give him 35mg of Benadryl a day and see if that helps. That's what I'm doing, and I think it might be working. I'm even using the generic brand of Benadryl. I have also changed my dogs food to an all natural food, which is expensive, but he loves it. Another thing we do, is give our dog Vitamin E every day. I'm not sure which of these things have worked, but it's worth a try. Even though the food is a little expensive, he only goes through a bag a month, and it's a lot cheaper than the medicine the vet originally gave him. The food we give my dog is Natural Balance, and it doesn't have all of the fillers that most brands have, which is really bad for the dog.
