I think King Daniel is right, it sounds like you have a bootleg set.
I have the offcial 2003 releases, and on a re-watch last fall (on a DVD recorder that I did not have back then), all the disks were fine.
My issue with the sets were the cheap plastic used to house the disks. They mostly fell apart within a year, except for season seven, which finally fell apart during my recent re-watch. But, I solved that problem with a few hundred miles of scotch tape.