You can have the blood drawn and sent to someplace like Spectrum Labs or varl for a blood allergy tests. Some vets don't like them, but they were incredibly helpful for both my dogs.
I don't think you're going to figure this out unless you go on a homeprepared trial. It could very well be the chicken, even though seems better on the Iams. Until you have control over the ingredients, it's all a shot in the dark.