Or maybe just some dirt in the slots connectors?
Oh, one more thing: always have the card protected and always release it with the little symbol in the menu-bar too, not only by the icon on the desktop. I’ve experienced my PowerBook crashing when not doing so, and it might be the reason for reading errors as well.
I doubt that it’s your harddisk, since that will report I/o-errors when damaged, it won’t introduce arefacts silently.
Regards,
Uli
Author of “DVDs gestalten und produzieren”, a book on professional DVD-authoring in German.