I had the same framerate issue. It got choppy always when that Briarheart dude found me. Same place, same enemy. Other than that I've had no problems. My game time was about 30-40 hours at that point. Solved the problem by sniping him from the hill. Somehow it didn't get choppy when I was up on the mountain.
That looks more like what happened to my game once on the PC. There was this guy who tagged along to help clear out a dungeon. When the quest finished, he stayed on the same spot, I guess the path tracking bugged out on him. If I talked to him the framerate was smooth, but once I started walking anywhere the framerate was exactly like in your video. Once I got out of the dungeon the framerate went back to normal. This happened like on my 6th save over 100 hours of play, so it wasn't something permanent.