I had same issue as you have . After waking up from sleep there is no stammer and morning also very less stammering. The reason is your tongue is very relaxed in morning hence making pronounciation of words easy. But in later day as you have done lot of talking it gets pressured and you start stammering. Easly curable but it won't come in a day or two . It needs months 1-2 of intense pratice and additional 1-2 year of daily 10 mins self practice and some real life practice to make it your second nature.