The images we create present themselves in mind-body states. In other words if you see yourself on the edge of a cliff about to fall off thatll send your body into a state of panic or if you imagine a calm peaceful meadow with you lying on it that'll send your mind-body system into a state of tranquility. These states dictate our behavior so when you smoke you're in a calm state not focused on what you're saying etc however when you speak or block you're in an intense state of anxiety right? That state sort of takes control over you. That's because the images and words we use in lur mind creates this states based on many factors. I'll take it one step further and really blow your mind. When you think about blocking and imagine that grey dull picture, notice the thoughts that come along with it. It takes a while but it'll probably be along the lines of "I have to control this" or "I can't speak normally" or "remain calm" etc then on top of those thoughts you'll have other thoughts about those thoughts and so on and so forth. What I'm asking you may seem simple but there's unconscious processes going on in your head that you're not presently aware of that are dictating whether you stutter or not. Again I know how it sounds but this is just the beginning of it.