As a piano player and person who stutters, I've experienced something similar to what you described while playing piano. It usually happened when I'm about to play a trill or grace notes. But it would happen right before I'd play the first note, almost as if my body froze up. It was really difficult to "unlock" this freeze response. I would say this is very similar to a stutter. It's almost like your hands are stuttering, or more correctly phrased in the stuttering community, 'blocking'.