T.S.
Is it a stutter (stuck on a sound "t-t-t-t-t-t-t-t-table") or a stammer (stuck on a word "whatwhatwhatwhatwhatwhat's for dinner?")?
A stammer is REALLY common in kids this age as they start to have ideas they want to express and are thinking faster than their little mouths can form the words.
Either way, if you're worried, call the pediatrician. I GUARANTEE they get this question all the time and they'll know whether or not it needs intervention.
HTH
T.