When you wash your hair, are you paying attention to your scalp? You want to scrub with the pads of your fingers, until your arms want to fall off.
Try using Selsun Blue Naturals, the one with 3% salicylic acid. The SA will exfoliate your scalp.

Before shampooing with the Selsun Blue, take a boar bristle brush (Sally's beauty supply) and really concentrate on your scalp and give it a good brushing. That will give you a good place to start.

It could be seborrheic dermatitis, psoriasis, etc.