So, for me it is that I can not really decide "what to say next" until I have gotten "what I'm saying now" out of my head and onto the screen.

That is, if I can keep an even pace where I'm typing at roughly the speed I'm saying what I want to type, then I don't do this stutter that often. However, if I am constantly getting adrift in how fast I'm typing versus how far ahead I've thought of what to say, then I stutter like crazy.

I'm assuming it has a lot to do with how you "speak" in your head. Some people (myself) do so verbally. Some do so visually.