This happened to me after I got my game back from the person who had borrowed it (for nearly two years). I couldn't get into a certain area in Treno during Disk 3, and I had to borrow my friend's copy to get any further.

I'd suggest investing in a Disk Doctor or some such device to resurface your old disks, if several of them give you trouble. If it's not a big problem, or if you don't want to spend the money, you can try to find a friend to lend you his/her copy, like I did.