Yeah, I would try submitting a bug. For the time being I'm sure you could probably ask someone for a fresh save point to get you by that spot. It might work. Not sure.
I'm not sure how good or bad the Ati drivers are on a Mac, but I know on linux when I was using my Radeon 4850 I was having a lot more issues then with my Nvidia card. It would often freeze up my computer after about 15 or 20 minutes of play and I would have to hard reset. Maybe your being plagued with a similar issue. Make sure your driver is the latest one.
Once again, just spit balling here. I'm sure you likely have already tried that.