It's a classic sign of a bad combo drive. I'd try several DVDs just to make sure the one you are attempting to play does not have a defect, and also I'd try to see if it works with data DVDs...those are generally better for testing, because they simply show up as an icon and don't launch the DVD player, etc. But if it's unreliable playing data and video DVDs, most likely it's just a bad drive. Keep in mind also that early combo drives were very touchy and it's normal for the drive to dislike a DVD here or there. I'd return it to the source on Amazon and ask them to swap it out. Your computer (and any drives that are out there) are more than 10 years old, so regardless of whether the seller states "barely used and in good condition", it's going to be hit or miss.