I have had alot of experience with allergies in my Golden Retriever. I would strongly reccomend seeing a veterinary dermatologist if you have one in your area. They can do allergy testing and reccomend diet or other things (allergy shots) you can do. In my experience, even very excellent general veterinarians are not as good as a good allergist/dermatologists with this problem.

In the meantime you could try bathing your dog with an antibacterial shampoo. Dogs with allergies tend to get little staph infections in their skin, this is why they itch. In my experience, benadryl doesn't really help much, there are other medicines that are more helpful (amytryptilline, prescription only, for example). Diet may also be a factor, and the dermatologist could give you advice based on the testing done.

Good luck.

Jake'smom