OK, I'm trying my second trial run on my UDS to see if I got all the bugs worked out. It appears that the culprit causing my temps to drop and not go back up was the charcoal basket sitting directly on the terra cotta ashpan. Air was not able to get to the charcoal from below at all, so when the stuff on top burned and started to form a little ash the stuff on the bottom was unable to ignite. There were probably 18 - 20 unburned briquettes underneath the burned.
Got rid of the ashpan all together and raised the charcoal basket about 3/4" to make sure I'm getting good airflow. Now I only have 23-3/4" clearance between the charcoal grate & bottom cooking grate, which should be fine.
Anyway, I put another 9# bag of briquettes in the basket and poured 1/2 chimney of lit coals into the cavity in the middle. Took 3 minutes to get up to 225* and it's now coasting at 229* for about 15 minutes with one 1/2" inlet completely open and the 1/2" ball valve cracked open about 1/3 of the way.