
Hazel,
With food allergies the problem tends to be persistent because the dog is continually exposed to whatever is causing the problem. If you dog's skin flairs up every couple of months regardless of what you feed its most likely not food that is the cause; IMO the more likely culprits are things like dust mites, molds (outside), pollen or chemicals present in things like washing powder.
I know it can be expensive, but my recommendation would be to try allergy testing.
That said, if you still want to switch foods its best to try one with ingredients your dog has never had before - for example, a food based on salmon and potato with no meat and no rice ... and no artificial anything added.