We had Chester tested for allergies and found that he was allergic to several foods (beef, chicken, turkey, lamb) and tons of other grasses, weeds, trees, etc. We now have him eating Nature's Recipe Venison & Rice, and he gets monthly allergy shots. We also occasionally give him 1/4 of a Temeril P (sp?)tablet, which is a combo of steroid and antihistimine. Things are SO much better now. He was literally chewing his feet of for a while.
Many vets suggest the no corn, no wheat foods, but the culprit is usually the meat product in the food. And testing is so much easier now. They simply take blood and test it (this allows for the food tests).

Good luck!