I'm w/ the out-of-round crowd. ("I'm In w/ the Out Crowd"?!)
I recently had problems w/ my 18.5 WSM a few cooks ago- runaway temp. to the point where I closed all 3 intakes and the thing ran at 250 for hours.
Turns out the body is out of round, and when it doesn't sit/seal properly in the bottom section there's a decent-size air gap.
I haven't addressed it other than to make sure the body sits correctly when I cook- no more problem.
1/4" Cimarron offset & Webers galore. REAL men drink ALE.