I believe it is psychological and revolves around fear and anxiousness. Some vitamins have helped me, such as B12 and more recently, K2. A while back, while I was in Korea and fasting to lose weight, I discovered that after eating a certain Korean soup, my ability to speak was completely unhindered. It was just a simple leek soup with a lilly seaweed and soy. I would really advise anyone who stutters to experiment with what they eat. Generally, eat healthy, unprocessed foods. But for experimentation, try monofood diets to see exactly what helps the most.