Well, I thought there might be air leaks too, but when I only have a few briquettes burning, the smoker responds well to intake damper adjustments. I can increase, maintain, or decrease temperature easily, as measured at grate level with a wired-remote digital thermo probe. It's only when more and more briquettes start burning that I lose the ability to control heat.
If I did have air leaks, I'd think they are from around the ash drawer. I did what I could as far as welding on it, and there is a good seal between the drawer frame and firebox. I also welded the firebox halves together on 2 sides (the other 2 sides are true to each other with no gaps) and seal the gaps around the movable part of the ash drawer with aluminum foil before firing up for the day.
I do have small wisps of smoke from both lids (firebox and pit), could these be significant enough to cause this? I wasn't sure it would affect it since they are above the flame whereas the ash drawer is below it.
I got to thinking last night, maybe it's the way I'm burning the charcoal, as well as still having some air leaks. Yesterday I did some country ribs, and tried something different. I covered just the bottom of my charcoal basket with a layer of unlit briquettes, and put 8 fully lit ones on top of the unlit, as far to the left as I could (left side is farthest from the intake damper), yet after 20 minutes of this, half the charcoal was burning and temp was climbing again. Seems that no matter how full of a charcoal basket I start with, the bottom-most layer seems to light off which of course gets the stuff above it going right quick.
Dave, I've given lump charcoal a try and had the same results BUT that was before I did any modifications to the smoker. I'll give it another try!
I think what I'll do is connect a shop-vac to my stack and pull a small draft thru the smoker, while moving a smoldering piece of wood around to see where smoke gets drawn in. That should help out a lot.