Try deleting the game data. Go into the XMB menu. This is the menu that you see when you turn on your PS3. Select Game / Game Data Utility. Select the game and press the triangle button. Select Delete and then press the X button to confirm. Once this is done, reinsert the game disc in the drive. You will need to install the game data to the PS3 again. This should not affect your saved game as it is stored in a different folder.