Mine is 100% mental. Any physical aspect is a learned symptom. My brain knows the exact word I will block on, then several words prior to that word, my brain starts stuttering. It knows the problem word before my conscious self knows, but I’ve been trying to keep up lately and tell my brain ways to bypass that problem word, thus avoiding a stutter. If I, consciously AND subconsciously, could stop thinking of words, then maybe I would flow through sentences more fluently.