Where did you place the Maverick box temp probe? Are you sure it's not touching the cold meat, or sitting just above the meat? You may be getting false temp readings. Set the temp at 240 and look at the ribs after 5 hours. If they're not done, go longer. You very well may be getting improper readings. Let's see what the results are, without hyper-analyzing the internal temps during a cook. This is not your first cook, is it?