Sounds like a corrupt save file to me...
There's a few threads with saved game files on here, download one onto a USB stick and overwrite your current save data. Make sure that you get a save file that matches your DLC. eg. If you've bought the DLC you'll need a save file that also has the DLC.
I reckon this will probably sort out the problem, although I'm guessing. (educated guess tho!). It's got to be the save file because it works with a new user on your ps3, so it's not the disc or the ps3.
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