There are a few methods for igniting and getting up to temp, but I've had great success in only lighting 8 to 10 briquettes, placing the basket in the drum. With all air inlets open, I close one cap at 180*, the second at 200* and then close the ball valve to half throttle at 215* and it'll settle right in at 225*. If you're using this method or a similar variant, I think we can eliminate this as a possible issue.
I'd recommend giving it a couple more runs (empty or with some inexpensive meat) and see if your controlling the temps gets better. Sometimes I think it's just a matter of becoming more familiar with what you're working with. When I first started with the UDS I had a few panic moments, even wrapped an old blanket around the lid in a panic...lol It ended up my issue was from lighting to many coals.
Also, keep in mind that your actual temp out over the coals could be 30 to 50 degrees hotter than what you are getting from your short stemmed thermo. I tapped a hole in the side of my drum at grate height to use my digital probe near the center of the cooking grate for a more accurate cooking temp reading.