Is the focus of attention what is making your stutter. When you are "improvising" your attention is on the activity you are doing or the goal you are trying to achieve. You don´t focus on yourself but on the external. When you have time to mentally "rehearse" you go into a fear induced state. It functions as a sign to your body that is time to block. A trigger. You focus entirely on yourself or what you think that the other person will think of you. Here are my two tips: 1) Change your focus of attention when time is given. Pick up a pencil, sing a song, whatever. 2) Get in touch with your fear-state and also with your calm-state. Learn the diffence between the two and the strategies your mind does to get you on one and the other. In time you can learn how to switch from one state in to the other at will.