Why does Mirror's Edge go choppy and unplayable on occasion?
It's likely to be an issue with NVidia's PhysX engine. I had a look around and it's noticed that PhysX does not play well with a lot of ATI graphics cards.
I had the same problem and after a bit of searching I found that this works on my XFX radeon 6870 card.
go to options > video > and disable PhysX