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.
I was going to post a topic about the packaging the other day but figured it wouldn't get much traction. Still, it seems like every time I touch the set the packaging breaks even more. I'm almost tempted to buy the blu ray sets just to get better packaging because the stuff they used back in 2003 sucks. My Season 2 set is pretty much done in terms of how much the packaging has torn off (And I'm pretty gentle).