Same here! I stutter the most when I'm with people who tend to be rude or unpleasant, especially if they are some sort of authority like professors in college or in school. It's not like I'm afraid or something, I just don't feel comfortable and it starts to be very hard to speak. I don't have any good advice and I don't even know if you meant that, but if you did, I wrote this just so you know you are not the only one. Good luck!