Some 'backup' discs no longer work

[charminster]

Playstation PAL, about three years old, chipped.

Over the last few months some 'backed-up' discs no longer work, either sound is very jumpy or game wont start or locks up.

Have noticed it is worse with CD's that have a home printed label put on?? but even when peeled off still have problems.

All these discs did originally work fine.

Original discs are better but sound is still 'jumpy' on some of them.

Have very carefully cleaned lens using suitable spray etc, bit better but still problems.

Any thoughts much appreciated.