Christopher is absolutely correct about Nimoy's stilted delivery on looped dialogue. Even Sir Patrick Stewart, who was in DUNE, EXCALIBUR and other Classic Motion Pictures before TNG - whose acting chops are most impressive - gave some very awkward readings, whenever he had to overdub himself. In the first couple of seasons of TNG, in fact, this is quite pronounced. So, it's not like Nimoy is above a bad reading - just listen to his voice work on TAS. It's atrocious! And by the time TMP came around, nobody had to work that hard anymore to "sell" the show, as they had with the series.
STAR TREK was established and making a fuss over how Spock enunciated to convince people that he was from another planet just wasn't necessary anymore. So yes, Movie Spock is played differently. And starting with VOYAGE HOME, Nimoy was evidently quite bored with the character, altogether. I have to agree completely with what others have said, too, regarding Lenny's dentition - it screwed up his mode of speech and the shape of his face, somewhat. In any case, Nimoy's ADR work has historically been mediocre to very bad.