Same for me. It happens more when I'm stressed, uncomfortable, or trying to pay real close attention to what I should say. Happens more at work than at home, for example, when I'm talking to a customer and I can't say anything that can be misunderstood. I'm often uncomfortable talking to them and stressed about what I'm saying or what I need to say to them. I also noticed I stutter a lot when I'm recalling something from memory. Like a quote from a movie or an exact quote that someone said. That's when I pretty much lose my ability to speak.