What I need is some kind of telepathic typewriter

I seem to have a great talent for writing, but only when it comes to composing bits of the book/website/exciting superhero comic in my head at any time when I'm not sitting at my computer and typing. When it actually comes to writing it down, it doesn't work anywhere near as well. I should make some kind of resolution to sit down and transcribe what I'm thinking about as soon as I think of it, because I have a habit of sitting down and mentally composing huge long essays without bothering to write anything. I sometimes suspect I'm not a natural writer. Natural writers are the ones who write.