How to beat the last part of the last level on Halo 3?

I am on the last part of the last level where you are driving in your warthog, and you are on this big platform thing and you start of having to wheel around 2 corners with some flood hanging around, then you have to keep driving while some of those little aircraft things shoot at you with their lasers. You keep driving while some of the platforms continue to fall. You make this big jump onto this smaller area and then there is just a little ledge on the side. I follow this all the way down until i get to the platform I am supposed to go on and when i start walking up it ALWAYS falls. I'm always sooo close. Please tell me how to get past