falter, stop, stumble, hesitate, pause, block one's utterance, stutter, repeat oneself, sputter, halt, hem and haw.
"stammer." YourDictionary, n.d. Web. 18 May 2018. <http://thesaurus.yourdictionary.com/Stammer>.
stammer. (n.d.). Retrieved May 18th, 2018, from http://thesaurus.yourdictionary.com/Stammer
To introduce involuntary repetitions and pauses into one's speech:
A speech impediment marked by involuntary repetitions and pauses: