I've witnessed this problem more than once before, although I am unsure of what the cause is I can only guess it is because of the constant turning off of plugs to my xbox, just like how if you are offline and set your xbox's time it will be reset to 2005 if you turn off the xbox by the plug.
So anyway to get to the point, if you begin the game and then it says 'Loading downloaded content' but then freezes (you will know when it freezes because you cannot hear the music in the background, also pressing the 'home' button on your xbox won't show up). Then all you have to do is simply go back to your install disc and re-install the DLC...what it has done is completely erased your DLC despite having it saved in your storage section on your xbox.
The install loading screen will pop up and seem to take a while, there are times when it won't freeze but it will take a very long time (the average wait time is 5 mins) if it takes up to 10 mins or more then I suggest you turn off your xbox and then retry, I did have this problem and then after my 3rd retry it loaded pretty darn fast.
Re-install the DLC and viola! it should now work.