You need more draft, If I were trying to tune that monster I would make the stack at least 2 or 3 feet longer to induce more draft.
You could also try placing some steel tuning plates over your hole from the firebox to the cook chamber, below grate level and slide them around making gaps until you can get the convection down.
You could probably use a bit more intake size as well.
So here we are in my paisley crib, what you want to eat?
"Ribs"...ah, Latoya, I don't serve ribs.
Better be happy that dress is still on,
I heard the rip when you sat down.