Another thing I just noticed. You mentioned using 8 lbs of lump, how are your starting your fire? I only drop 10-12 lit briquettes on top of a full fire basket of tightly packed lump with wood chunks buried within the pile of unlit lump. It's called the minion method. The fuel will slowly burn down, providing a very long and very clean cook.
May be a combination of both factors.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. Weber SJS. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)