Oh duh. Why'd I not say this first? Haha sorry- I know the problem.
In Halo 3 you are allotted 151 custom content items. 51 of these are screenshots. The other 100 are for maps, gametypes, and films. Unfortunately, DLC maps count towards this 100 limit. So when you are close to the limit and download DLC maps, the DLC maps won't actually work because you have too much content. The solution? Delete at least three (or more if you've downloaded/created other files since then) maps/gametypes/films, one for each map (each map count as one- a mappack isn't one in itself. So all DLC maps automatically take 1/10th of your limit, which sucks).
Delete those from your Xbox. Then you should be fine.