If there are more characters on the screen than the number of shadows you have selected for it to draw then you will start to see the choppiness.
You may also wish to turn DOWN the settings relating to quality of the shadows, but I would only change one thing at a time.
NOTE: Dont post to this thread saying it didn't work for you. If it didn't work then go try something else.
I am not claiming this will solve all problems for everyone. But I have learned from many years with Morrowind and Oblivion that this will make a major difference for some video cards (not sure which, so just try it and see if it works for you.) Hope this is helpful to some people.
