We'll first have to define what we mean by "psychological" to get this line of thinking off the ground. For instance, psychological as opposed to what? Something "purely physical"? What would this even mean? We're whole organisms with loads of interactions with our environment at all times, our attention spreading out to all manner of things. Demanding that a certain phenomenon necessarily must belong to some more or less fitting category of existence seems unnecessary to me. So, I see no reason to try and define something like stutter in any narrow way. What's wrong with it being multiple things all at once? We may even eventually find out that there's variance between stutterers that makes any general classification pointless.