Manual saving for Tomb Raider Underworld on XBox 360?

The worst thing about having it on this platform is it will only save if I finish a level. So if I were to quit in the middle of a level I'd have to start at the beginning.I've beaten the game, however, when I go back through in Treasure Hunt mode it never finishes the level so I have no way of saving my progress when I collect more treasures or relics. Does anyone know if I can turn off auto save or if the only way to make them count is to replay a new game?