Hi there. Reading that you speak fluently the first 2 hours makes me think your stutter is mild, which is great. This is coming from a person with moderate to severe stuttering and would have difficulties after 15 minutes speaking fluently (or close to fluent I guess). For why this is happening, I think it’s a common recurrence with stutterers as you need concentration to focus on your skills that make you fluent (whether those skills were self taught or implemented by a speech therapist) and after a while of concentrating, your ability to concentrate further wears off and that’s when your stutter begins. As I said, for me it takes 15-30 minutes, for you it’s 2 hours. I read a study that students can’t focus on a lecture for more than 45 minutes without a break. So I think that’s the average concentration span for everyone.