isn't blocking also called stuttering? I have stuttering since I was very young and in the early days i used to stutter alot but now as I have grown up (currently 18 ) I gradually made a habit to block words instead of stuttering maybe because blocking words is not as embarrassing as stuttering. Even now when I have difficulty in some words I block those words and try again but rarely stutter like I used to do before. So I think for me blocking words has been involuntarily developed by me to replace stuttering.