This may be a stupid suggestion, but have you entered your online pass code?
I only ask because once I deleted my game files trying to fix a problem I was having in SP and when I tried to play the MP again, I got the error syncing levels message. I entered the passcode again and all was well.
if none of this work your best choice ( and last) is to buy the game digital . i was having the same problems u r not being to join a single game, game wont start up etc. it solves all of those expect for the game freezing