I vote air flow issue. Have you tried leaving the firebox door cracked open and controlling temps by size of fire? Many people with stick burners never close the fb door all the way.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)