My old boy has the same problem. He seems to have some sort of seasonal allergies. I had asked the previous owner and he said he is allergic to a fragrance that one groomer used on him, and had some seasonal allergies in the fall.
I had to get him a fragrance free shampoo and conditioner, and that did the trick until fall struck here. He's obviously got something seasonal going on.
When he starts in, I give him a benadryl and that seems to help a lot.
However, once he starts, it turns into an obsession pretty quick. I've started trying to give him a kong with some peanut butter in it, after I give the benadryl. That seems to get his mind off the itching and incessant licking.