This is incorrect and doesn't apply to all PWS. I'm a severe stutterer who stutters in all situations, every day, on every word. I do not have days or situations where I am "fluent" (though I can be relatively a tiny bit "more" fluent once in a blue moon within a very stringent, narrow set of circumstances - which aren't consistent). For me personally, my stuttering seems to be 100% neurological. Saying that all stutterers stutter because of "psychological reasons" is just perpetuating the stupid myth that stutterers suffer from their condition because they're anxious or lack confidence or whatever else bullshit. Also, there is evidence that singing or whispering entails a different speech-motor system than when talking normally. Though I'll have to dig up those particular studies as I don't have them on hand.