Why won't The Covenant level in Halo 3 load?!?

Okay, I'm trying to get the Marathon Man achievement and I need the Covenant to do so. It loads up to 20% then goes to 0% and tries again, then fails and it says "Players failed to load content"...???? I've done this level like 6 times, WHY NOW?? Someone please tell me what's going on and how to fix it...please! Thanks