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Looks like the last bullet
Looks like the last bullet under optimization may apply to you. There are also several tricks you can look up for deleting hidden data on your 360 that can become corrupt. I might start by deleting my XBL profile, deleting COD4 game saves (or copying to a mem card), and deleting all COD4 patches (look up the controller button sequence to access this in the system blade). Then I'd restart COD4, apply patches, and then recover gamertag from XBL.
That might be a goatrope, but it also might work.