Most I'd name have been mentioned, however I must note that after umpteen viewings I've finally gotten picky/tired about the following;
The Outlaw Josey Wales - once he meets up with Sandra Locke I'm out, prior to that still in
Goodfellas - first 3/4 until Tommy gets whacked
Jaws - I need to see Quint's speech, if I'm in before that then will watch till the end
Stripes - This one's obvious, the first 3/4 of the movie is great, then goes off the rails after the graduation
New
Rocky (sequels I can watch scenes check in & out, but original is a movie - I'm all in anywhere)
Officer & A Gentleman - Only if 'I got nowhere else to go'