My stutter can be severely worse when I am feeling under pressure to speak faster. I always feel I am speaking too slowly when telling a story, mostly just because I am uncomfortable when the attention is on me. When I'm trying to tell a story, I know I am thinking about the sentence ahead of what I'm saying, because I'm trying to tell a story quicker than I should be, which makes my stutter worse. For me, this is just social anxiety. I'm aware of it, but it's still hard to control when I have all eyes on me and I'm attempting to tell a story without stuttering or messing up my speech. Maybe this is your case, as well? The longer the story, the more pressure/the more time the attention as all been on you.