I would also ask if you immediately load another save on a different level, does the issue go away? Also, what are the specs of your console (model, board, etc) and is the game installed to the HD or running from disc?
Hmmm that has happened to me on Gears of War 2, Considering DNF also runs of the Unreal Engine (an earlier model) it may be a Engine issue.

i never had this problen
yer it only happened on this on game
yer i can its load another save just that one level so far but it keep happening on that level and know is running of the disc it the new black 250 GB that all i know