I experience the exact same thing! When I'm reading aloud by myself, I barely stutter. But then when I read aloud in front of other people, I start stuttering and I get really nervous. It was really bad in my final year of highschool, where we had to read out loud in class for English class. I couldn't even get through one sentence properly. I think this was due to the stress of final exams. In uni, it improved a lot and I could get by for the most part reading out loud in front of others without stuttering. But recently, it has started coming. I think a lot of it has to do with stress and just knowing other people are listening.