What Causes Recurring Itchy Rash All Over The Body?

I have a rash all over by body it's itchy and I have been to 6 dermatologist and they don't know what it is. It hurts. Plus I brake out in two different kinds of rashes. I have nothing on my face but my head, back, arms, legs, butt, top of my feet, under arms, neck, chest, stomach. But not all the time it comes and goes, but the other rash will go but it comes back. The other rash stays! If ay one can help that would be GREAT! I have had this since 2008.