I have this on my MBP which I spilled water on :/. I suspect ita the drive going bad to be honest, some DVDs, both burned and commercially bought DVDs, just get rejected, or never seem to spin up properly.
If you are in warranty I suggest you try and get apple to fix it, else I am not sure what to suggest. I'm replacing my machine anyway but you'd need to look into the costs of changing the drive if you are out of warranty, and that may become pricey.