Has anyone had issues with the superdrive not being able to read certain discs? I have noticed that on many occasions my superdrive does not read or burn cds (yet it can for the most part read dvds ok however burning them sometimes doesn't work)...does this sound like its time to purchase a new drive? I've also read that it might just need to be cleaned...the computer is out of warranty so I haven't taken it in to get it fixed just yet.