It is hard to say what is causing the problem without doing a physical exam. The symptoms could be caused by a variety of things such as worms, poor diet or poor digestion, mites, or hormonal conditions. I would suggest you bring a stool sample to your veterinarian to check for parasites because parasites can rob the body of nutrition and cause skin and coat problems. You might want to try our homeopathic product Derma-IonX, which is beneficial to the skin and coat. It aids assimilation of nutrients in the diet and can help the skin to heal. Also try to feed a good quality food and add digestive enzymes. A little bit of fish oil supplement orally is good too especially for dry skin.
To remove the dandruff you could try our shampoo which is good at cleansing, soothing, and removing debris but can also rid the body of mites and fleas. Please let us know how you do.