I thought I was the only one! I play guitar and started lessons with a very experienced teacher. He had never encountered problems with people who stutter before. He told me everyone has problems going fast at first. You tense up because you need to make rapid movements and these movements are done at a certain rythm/pace. It's difficult for everyone and not just me, is what he said. He gave me some rythm assignments which all were, start slow, and build up. This worked flawlessly. So in my experience, you can overcome this, and it is supposed to be normal.