For this type of assumption to work, your hypothesis would be best for baby babble or stroke patients (with aphasia). I say that because involuntary speaking is not a thing. In order to form a thought, and articulate it into actual speech with sound, it will never be fully involuntary. Your preface here is akin to equating how lungs operate with involuntary breathing, or your heart pumping blood. It happens with a 99.999% involuntary action. Stuttering on the other hand comes with speech, which by itself is a multifaceted process. There is nothing involuntary about it. Under these guidelines of being on auto-pilot, what kind of speech is present? You have to form a thought, articulate the thought into speech, and then speak the speech, and only then you may or may not get a stutter. There is nothing involuntary about it. Even if you repeat yourself a thousand times over to assume the idea of auto-pilot, are you really on auto-pilot? No. You have told yourself to repeat yourself over and over again. And before someone gets snarky and says what about getting out of breathe for your lungs or having A-Fib for your heart, those are outside influences impacting an otherwise involuntary action.