So he didn't recall stopping his camera. Read his grandaughter's book. You might learn something.
Zapruder's granddaughter gives the following account of his filming (this is from her book):
Zapruder said in the Clay Shaw trial that the film shown to the jury (multiple times) was the same one that was developed in Dallas the day of the assassination. The trial film was one of the copies made by Time/Life of the original Zapruder film (which they had) and that was subpoenaed by Garrison. That copy that was shown in the trial was later bootlegged and released to large parts of the public.
From the trial:
Q: Mr. Zapruder, from having seen what was projected on this film, can you tell the Court whether or not it appears to be the same as you viewed your original film on November 22, 1963 in Dallas, Texas?
A: Yes, it does.
It seems obvious that if large parts were cut out or changed that he would have noticed it. Maybe not but we have lots of other evidence that is, for me, persuasive that the film wasn't deliberately altered.
The rest of his testimony is here:
http://www.jfk-online.com/zaprudershaw2.html