Ok,i've owned the iq 120 for years now, your set up is fine lite only the middle of the pile of lump about the size of a baseball, open the top vent enough to fit the steel braided cable from the probe. Dial in the temp just below your target, once it gets there dial in your preferred temperature. once your there as long as your not opening your Keg to often it should lock it in solidly.
Hope this helps
I did as recommended and the keg stayed lit and under 250 for the duration of the cook. I had the temperature probe in the gap between the diffuser and the wall of the keg. It took an overnight cook to bring two racks (7lbs) of beef ribs up to 200. Pulled with a little oil were dry but tasty. I will try again and set the temperature a little higher.