The science behind randomly being unable to speak words is that dysfluency is basically a type of social anxiety as opposed to a speech disorder. Note - everyone has repetitions, and blocks are wayyy more impactful than repetitions. Blocks happen because we are trying way too hard to hide our repetitions. After you’ve had a period where everyone is very aware of your dysfluency, there’s less “need” to hide it - so you “magically become fluent!” This doesn’t last because you don’t have the few other tools to stay that way.