If there's any way you can let your professor know ahead of time about your stutter, if they don't know already, I'm sure they will be understanding. Especially if you have a really good grade, they know that you have a good grasp of the material. I ran into a similar issue in my public speaking class. It was the final project, and I had a really good speech written out, and I memorized my talking points. When I presented my speech, I blocked pretty badly, and had to cut out certain parts because I couldn't get them out. I actually almost ran over my time limit because my blocking was wasting time. It was awkward for the class to sit through, and embarrassing for me because I know I sounded unsure and dumb even though I knew the topic quite well. I got a B and the instructor was really nice about the while thing. It sucked for a minute but the most important thing is that you persevere.