I am smoking a 10 pound pork butt (picnic end) on my WSM; started at 7:30am. My good thermometer quit working just as I was getting started, so I'm using the built-in lid thermometer and a meat thermometer in the butt. The temp at the lid has been holding 220 - 245 (I'm new at this). The butt hit a plateau of 180 degrees at 4:00pm, and was still 180 at 7:30pm; 3 1/2 hours. Is this normal? Is something wrong? I have checked the temp with an instant read thermometer and they agreed. Please help. The butt is still on the smoker.