Just speculation, but maybe cliched curses like 'fucking piece of shit' is when your brain is in its comfort zone, just like when some of us add works like, ummm and ahhhs, or 'actually', 'I mean' etc. Try thinking on your feet and coming up brand new curses, and see if you stammer then? The added pressure of coming up with something new and whether the people around you find it funny *might* make you stutter. (I can't believe I've asked you to intentionally try to see if you stutter. Dammit.)