Itching while he is turned out, but not turned out 24/7, does not necessarily eliminate stalling as the problem. Have you tried pasture boarding him for a week or 2 By himself & with another horse? If he has established negative activities as a reaction to being stalled, he will not necessarily change the now ingrained behavior when he is turned out, partially because he knows he will be put back in a stall within a few hours. It sounds like you have eliminated all the usual physical causes, so what is left is mental causes, the same ones that can result in cribbing or weaving. Once a behavior pattern is firmly established, it is seldom dis-established overnight.