I remember an Aaron Franklin Video where he was cooking a brisket on that same pit. He mentioned that there was a Hot spot on the stack end.
A quick fix is to use the rack from an oven broiler pan under the grate slide it end to end until you find the stack ends temp drop and equalizes across the grate Start it at the stack end and slowly move it toward the FB. Small moves make big changes this is where a few Cans of whomp biscuits come in handy. The factory located thermo port is in the wrong place on that pit as well it is to high to do you any good. KNOW the Temp at GRATE LEVEL that is where your cooking at.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC