Assuming that the DVD plays okay in other DVD roms/players you could try updating your DVD ROMs firmware to see if that corrects the problem.
http://forum.firmware-flash.com/portal.php
If this doesn't help then it could just be that your DVD rom isn't working properly and will need to be replaced.
I have an old DVD rom which would only read certain DVDs whilst failing to recognise the discs with other DVDs. I guess the drive was failing as now it won't read any DVD discs at all but still works okay with CDs.
You say it reads DVD-Rs okay so maybe it is able to read single layer discs and not dual layer discs. Either way it is likely that you will need a new DVD rom.
Of course if you want to be certain you could try your drive in another PC and possibly borrow another drive to install in your PC but I would guess that the problem is your DVD rom.