What Causes Extremely Itchy Rashes On Chest, Arms, Back And Knees?

I have a VERY itchy rash on my chest, upper and lower arms, upper back, right hand and knees. I have suffered from ecxema for years off and on. I have been to dermatologists over the years but nothing seems to help, like lotions, soaps, etc. The rash eventually 95% disappears but I am having the worst episode right now. I am an 82 years old woman and am at my wit s end. Please help me.