It sounds like allergies to me, but I'm no expert. Try finding a really nice allergy shampoo. (Not one from Walmart or other super centers, but something from a pet supply retailer) and try bathing once a week with it?
My fiances sister had a dog that did that basically. She has lots of missing hair in some areas and some scabs. Allergy meds also helped her.
Maybe someone will have some more expertise in this area.
Also, what food are you feeding? Maybe someone will have advice on that as well.