I'm having the exact same issue, but it's not tied to the Dietmar fight (I haven't even gotten that far yet). This has happened to me 3 or 4 times already.
- Everything is working well/running smoothly
- Framerate suddenly drops through the floor (constant 5 FPS the most recent time)
- Graphic settings have absolutely no effect on this
- The only fix is to reload an earlier save
I just left town for the first time via the west gate, and as I said, I've already had this issue 3 or 4 times. It's making it extremely difficult to play, I may just give up for a while and hope some of the issues get ironed out in time.
