You are doing all of the right things! Curious, why does this professor not follow the accommodations you have been allowed? Can you shoot them an email and ask for 5 minutes. Your professor should want you to succeed. Lay it on too - "I am enjoying your syllabus and class. I am not sure if you are aware, but I have a speech disability called stuttering. There are some classes that offer me an extra 3 minutes for presentations. Would that be possible for me in your class? I want you to know I the material well for this subject. I just need more time to get the words out sometimes. Thank you for your understanding. I am trying. Sincerely, "