Itchy dog

My dog (large mixed breed) gets itchy every July/Aug. It does not appear to be fleas, perhaps an allergy of some sort. I've taken her to the vet a couple times and they don't see fleas either. One vet treated her for fleas anyway, which was expensive, toxic, and ineffective. Another vet suggested giving her benedryl until the itching subsided, which worked fairly well. It's July again and she's itchy again. Does anyone have any suggestions other than benedryl? I'm particularly interested in something I could give her starting in June to try to stave off whatever it is that bothers her. I feed her grain-free dry food moistened with herb tea and fish oil.