Why do I only stutter when I think what I want to say?

Why do I only stutter when I think what I want to say? I swear whenever I think about what I want to say I stutter and can barely get the words out. But if I don't think about it most of the time I can talk. It has nothing to do with feeling nervous or anything like that, it doesn't matter what mood I'm in. It has gotten to the point that when I have something I need to say but I think it first I just make random sounds in my head and for some reason it works. I don't know why it works it just does, well most of the time, sometimes it doesn't.