Honestly, this is really interesting to me. I've played piano for 18 years, and I've never felt this happen. I've since picked up bits and pieces of other instruments, and I've never experienced it on anything else either. It's interesting to me, because I always see music as my outlet from my stutter. When I play, I can say exactly what I want to. Probably my favorite feeling is being on stage and playing/singing, because it's so freeing. Which is why I'm confused about what your describing. Do you mean that your finger literally will tap multiple times uncontrollably when you don't want it to? Do you get blocks, where you cant move your hand? Out of curiosity, how long have you been playing?