I have a skin rash tht lookss lke eczema but it clears up and strts agn...on knees, arms, back and hair?

It can be a form of eczema that is contact dermatitis. Your body may be reacting to a specific food, body product you use or irritant that irritates the skin. It sounds like some kind of allergic response. If it was bacterial or another source, it is unlikely to just clear up on it's own and then start again.