Sentence:  The content of his words may <WSD>lapse</WSD> back into monotonous repetition , as if a phonograph needle were stuck in one groove ; only seldom is it so simple as to be a matter of his obviously parroting some timeworn axiom , common to our culture , which he has evidently heard , over and over , from a parent until he experiences it as part of him . 
Meaning of the word marked with <WSD> tags:  drop to a lower level, as in one's morals or standards