Jack Russel skin problems.

My Jack Russel has bad skin. Every now and again she gets really itchy and has bald spots appear on her back and tail. Until recently it hasn't got that bad, but now bald patches appear on her legs, and feet. She also rubs her tummy on the floor and chairs, and this has resulted in the skin on her tummy going black in colour. The skin on her itchy areas is red/pink. I have been brushing out all the built up scabby bits.
Also up until about a few weeks ago she has been scratching at her ears, they have gone inflamed and smell a bit cheesy. There is a bit of wax in there, and I have been wiping her ears with special doggy baby wipes for the ears, the redness in her ears has gone down, but she is still shaking her head, and scratching them.
Could she have a skin allergy, like dermatitis, or perhaps a mite? We haven't changed any chemicals in the house, and she has been de-flead.
Would giving her Evening Primrose help with the skin condition, and bathing her in insecticide shampoo, and using ear drops help?