Well what sort of CD's are we talking about, commercial, system software, computer recorded, and are the CD's CD+R or what? Have you tried running Repair Permissions in whatever operating system you are using, and if you have the original discs, run Apple Hardware Test extended over the machine. It also does happen that cheaper media does present problems and personally only use Verbatim discs. Also a good quality optical drive cleaner may come to your rescue.