What Causes Intermittent, Itchy Rashes On My Left Arm?

I currently have an itch, rash, on my L arm. It comes and goes. I have had it for the better part of the last 7yrs. It started strictly on my forearms, on both arms but never at the same time. But, like I said, it comes and goes. Now, for the last year, it is strictly on my L arm and it goes from my wrist to my shoulder. I have been to my family doctor, 2 of them. A dermatologist, everything came back negative. She said it was a neurological. I went to the neurologist. Once again the tests came back negative, except I have carpel tunnel. lol I went to an allergist and he is perplexed. Nothing seems to work. The only medicine that works is Hydroxyzine. I usually have to take about 200mg daily, it is the ONLY thing that has worked. Any ideas!!!!