Rash started legs, that comes and goes all over.?

I've had this rash for almost 5 months now. I've seen 3 doctors, and 1 dermatologist. None of them know what it is. It's a red bumpy rash that itches and spreads. It comes and goes frequently. It started on the back of my legs, then to my arms and elbows, shoulders, & neck. The itching has been all over my body, even the back & sides of my heard. I've been treated for scabies twice, so that's not it, and the way it comes and goes is not how scabies would act (per the dermatologist & doctors). I've been prescribed a type prednisone twice and that has stopped the rash and most of the itching both times, but Dr's don't want to keep prescribing this steroid. Does anyone know what this could be? I'll go weeks with no problems, then it flares up again. It's very unlikely that it's an allergic reaction because nothing in my daily routine has changed or has noticeably set off the rash, it is VERY random. And after 5 months, I think I would notice if it was something in particular.