I've seen this in a couple of apps (children's games and "The Crystal Key" in particular). Here is what I generally try: stop Classic, eject the CDROM, start Classic, and then insert the CDROM. This works 99% of the time for me.
Another trick is to copy the CDROM onto a disk image. This has the added benefit of speeding up what you're running, if it works.
However, no amount of fiddling would get "The Crystal Key" to work under Classic for me. Ultimately I had to reboot into OS 9 to play it.