Brad White

Not sure if anyone else is having this problem, but when playing the campaign on my Limited Edition version, I can get up to the airlock just after you fire the missile to destroy the covenant ship on the first level, but then the game crashes and sends me to the start screen without saving my progress. I'm not connected to live at the moment, so it isn't that. I tried three times, all with the same result. I got the achievement for unlocking the Chief's past the first time I played through, and it still shows as being achieved. Can someone suggest a way to fix this please?