Hello aeropa,
It's either a bad drive or bad discs.
Note that a DVD drive has two lasers inside, one for CDs and one for DVDs, and it is possible for one of the two lasers to go bad but the other one continue to work just fine. Have you established that the DVD drive can read both CDs and DVDs?
If bad discs, sometimes you can "revive" a bad disc by copying it with a major name brand utility such as Nero or Roxio/Sonic, and then using the copy. The copy process's error correction capability can compensate for areas of the disc that are
marginally readable.
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