Just a guess but the the ribs on the lower rack were absorbing the bulk of the heat and the probe was therefore not able to regulate. Another possibility was fire in the woodbox causing the char on the lower rack. Your 2D have the probe at the top? Next time stagger the ribs and allow the heated air to freely circulate. Or try using rib racks and stand the ribs up. Lastly, ensure your box probe is clean and free of stuff. When you have time, run your smoker with a brick of something similar to simulate a load and watch the light to see if your unit is cycling on and off.