All right, I had to think about this one a little and hopefully I can explain it clearly. *Strictly speaking* the phrase "a stutter" only describes a singular instance. The thing is, we often say "he has a stutter" which changes the meaning and makes "a stutter" more like "stuttering" because we've attached it to the subject ("he") and in doing so we imply that it's a more permanent, ongoing feature. I would encourage you think of "a stutter" and "stuttering" more like "a jump" and "jumping." We're describing an action in both, the difference is whether there's one or multiple. English majors/linguists feel free to tell me if I'm wrong here.