Surely this would indicate speech anxiety rather than a lack of ability to speak. I work in the Uk Police force and for example when I say O66 as a call sign, in the car with my colleague. It is clear as day, but when I press that button a feel the tightness and the anxiety, and then the stammer kicks in. I am just sharing thoughts here, but I really feel that it is an anxiety of judgment sometimes, ever feel you stammer more around different people?