It could be because when you in certain pressure situations you subconsciously think about what you are going to say before saying it. I find I am also in a similar situation where if I am 'scripting' my speech, where I basically tell a story or talk in a very scripted manner back and forth I don't stutter as badly. However I struggle the most in casual free flowing conversations, like when I have random casual conversations with coworkers not related to work.