*I've* never heard of anyone having an internal stutter. Just as singing and speech are very different activities, I would imagine that your internal dialogue is even less related to speech. When you speak to yourself in your mind, none of the muscles / mechanics required for speech are involved. Speech is a complex process when you start putting all of the mechanics of breathing, vocalizing, and annunciating (I'm not an SLP, so pardon me if I've omitted other components.) If I have any of this wrong, please correct me.