Doesn't happen at all to me. But then, I guess that's why I'm a writer. Writing involves the transmutation of thought into text. Without that process taking place, writing is impossible. It's like trying to start your car up with a full tank but a missing ignition.
It could be that linguistics isn't your forte- that you're not particularly locked into how language operates, how words lock into each other. In that case, if you're more comfortable with the spoken word, I'd probably recommend buying a tape recorder you can dictate ideas to.