Currently Being ModeratedMay 16, 2013 10:56 AM (in response to James Thomson)
It could just be that the optical disk is failing. You might try running the Apple Hardware Test according to this Apple document: OS X Mountain Lion: Use Apple Hardware Test.
Or take it to an Apple store and have them test the drive. If the drive is bad it I suggest getting a new drive but one with a drawer load rather than a slot load. A lot less chance of getting a jam.