I think it was because it was when he started 'singing' rather than relying on his beat boxing... I think it could be why he sounds a bit off rhytym to me in his live performance, possibly he was concentrating/focusing on his singing vocals which threw off his timing OR, like I said, maybe he had ear monitor issues and couldn't quite get into rhythm w/ what he was hearing.....
You can see how he doesn't really 'sing' the lyrics as much as sing w/ control....
according to
Wiki, his studio version: "Time of the Season", The Zombies' hit in 1968, reached number 67 on the Billboard Hot Digital Songs