I recently had problems after downloading all the map packs for COD black ops. at first we thought it was a problem with the download but that wasnt the case. If u are having the same troubles and do not have any corrupted files on ur hard drive, try getting your game disc re-surfaced. most game stores do this for a very low price. I had it done and have not had any issues since.
This is because despite the maps being a download, the key game data comes from the disc. If the disc is scratched or damaged your downloaded maps won't work either.