I believe the issue is, as you already stated, the cd. If patches doesn't help, then you could try to reinstall the game. If that neither helps, then it's the cd.
If that's the case, I suggest that you buy new one, in case you still want to play it. This is because even though you may get a save past to the current problem section, there is no way to tell what else is corrupted. Also, with asking people to play the game to the point, you either don't get the party what you would prefer, or at the very least not developed in a way you would have prefered.
EDIT:
Oh, and
here you can download patches... though I'm not 100% sure if the patches are same both for Win and Mac...
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"As we all know, holy men has born during Christmas...
Like mr. Holopainen over there!"
- Marco Hietala, the bass player of Nightwish