What are you using as a temp gauge? Those analog ones are usually WAY off.
When you start your UDS, do you open all of your intakes? If not, you should. Then AS SOON AS YOU REACH YOUR DESIRED TEMP, cap off all but one of your intakes. For me in our current ambient weather pattern, that puts my UDS right around 220. If I want to run higher, I open up another intake and gate down the ball valve.
You will find that the more you cook with your UDS the more stable the temps will be.