Ok, after spending some additional time with the game, it appears that it can get pretty unstable when there's a bunch of things happening on screen at the same time. (Painful in later missions.) When this happens, the screen typically goes blank, "minimizes", sound loops, and the only thing that can be done is killing the process. I ham-stringed it by restricting CPU affinity to just 1 core, just to see what would happen, and, while it lags, which never seemed to happen before, it also crashes much sooner with nothing really going on. I figure the two are related.

I ended up dropping from 1080p to 720p, but keeping graphics settings set to Ultra, and ran through a bunch of the areas / missions where it tended to crash, and everything seemed to be fine. Most of the time. This appears to only mitigate the problem. It doesn't fix it. I never had this problem with SR3. *knocks on wood*

If you've had similar experiences and would like to share what you did to work around them, I'm sure others would appreciate it, too. I know I would. (A little more comforting to know you're not alone when these things happen.)

As a side note: When it's not crashing, the game flies and looks amazing compared to when I played it on the ps3.