you haven't mentioned how much intake you are giving it. Franklin actually cracks his firebox doors open to allow more oxygen to get to the fire.
Many less than perfect offset COS cookers don't have large enough intake holes to allow for good draft. Easy solution is to crack the firebox door open and get a nice small clean fire goin.
On the smoker door, I'm sure it'll help to keep it closed. If there are signs of huge leaks in the door, a gasket kit might help.
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)