I most commonly stutter on hard consonants, like b or t. It almost always happens at the beginning of a sentence and rarely happens later in a sentence. When reading a large passage out loud, there is an ebb and flow that seems to occur, I'll come to a word that needs to be repeated or something like that, then start to build some momentum after the word was crossed, then a few sentences later take some time on another word, then build momentum again. Interesting though, I neither stutter when singing (Different part of your brain) nor when I'm reading in tandem with someone else. Just a few thoughts.