do you happen to be downloading DLC every few days? For some reason downloading DLC can cause some data in you game to be deleted. To prevent this do one of the 2 things below after downloading the DLC. 1) Once the DLC is done installing shutdown the console. Or 2) When all the DLC is done installing return to the main menu go to options>save manager>load and confirm you want to load. If you haven't been downloading DLC each time it happened then I have no idea.