I imagine you are certainly in a plateau. However, the temp reading could be misleading. After a point, you'll need to do other tests as to doneness. Wiggle the bone and see if you feel a little release when you tug it as if it might pull out. Reposition the therm in the butt to see if your reading is different. When you get beyond, 2 hrs per pound, get a little suspicious. However, completiton times are not always predictable.
Paul
