I have never tried lump in my UDS, but I also have a 12x2" stack on mine and I never tamper with it. I have three 3/4" air intake holes on the bottom of mine and two of them are ball-valve controlled. Your air flow is simply a little too high. You need to control your temperature with the intake rather than the exhaust on the drum. The draft created by the stack will still be limited by how much air is allowed in at the bottom.
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