Yes I know why. When you're "under the influence" your mind-body state is relaxed, your mind isn't focused on the problem, you're in a calm state. The good news is you don't have a stuttering problem. If you can speak fluently in one context you can speak fluently in all contexts. I wouldn't use weed as a vice for stuttering because you aren't actually dealing with the problem itself but the problem can be dealt with. When do you normally stutter? When you're about to speak to an authority figure? Perhaps when you're about to make a call? Your mind creates this intense state of fear and panic that presents itself physiologically. It's psychosomatic in nature but there is a solution.