I have the same problem. For me it is because I'm scared I'll stutter and that stress makes me stutter. It's weird but when I'm alone I speak fluently so thats probably the cause. For you to reduce it, it depends on what kind you have. There's 3 types, the repeats, the speech blocks(I have that 1)and making the 1st letter longer (example: mmmovie). See what type you have and look into tricks to control them. But wich ever one it is just try and be calm and relaxed when speaking.