Small sample size sometimes throws things off. Are the discs it wont read scratched? Alternatively, are you running a weird configuration where your DVD drive is on it's side?
Also, it could be that the drive is damaged enough that it is requiring a higher level of precision from the disc to be able to read them. I had a similar problem in my old PS2 (hey, it's still a DVD drive, even if it's not a PC) where it would play some games, but not others. It was that the laser was out of alignment, and some of my older discs weren't perfectly balanced, so they wouldn't spin flatly enough for the misaligned laser to read the disc anymore, or they'd only read them sometimes.
I'm still going with faulty/damaged drive.