When I first started smoking I had an identical reaction. My stutter would totally disappear. It was a good feeling. Eventually my body or mind or whatever adjusted itself and it kinda wore off. I still have more control when I'm high than I normally do but the stutter is definitely there. I have not kind of proof or research to back this up but my theory is it's at least partly due to speaking in a bit different way when you are high or drunk. It is kind of like not stuttering as much when you sing or speak in a fake accent.