stammer vs stumble over

stammer

verb
  • To utter with a stammer, or with timid hesitancy. 

  • To keep repeating a particular sound involuntarily during speech. 

noun
  • A speech defect whereby someone speaks with a stammer 

  • The involuntary repetition of a sound in speech. 

stumble over

How often have the words stammer and stumble over occurred in a corpus of books? (source: Google Ngram Viewer )