I literally have the same issue as you and i know now why its happening to me so the thing is the stutters have a idea that they have to use effort to get the words out so they starting to build up air pressure as you know when you block you can fell you chest is hard as rock and also your diaphragm so its just a habits and the thing with is its unconscious so you need to practice alot to cure it but trust me th result is worth every little work you do