Let me preface my response by saying that I do not have much experience with issues like this, but I would 'guess' that it's the player involved, and given that new players / burners are relatively inexpensive these days I would opt for a replacement unit.
Even if the replacement unit behaves the same with the same disks, you'll have ruled out the player being the problem (it's the only way) and you'll have a spare that you can use in the future.