How big are your chunks of post oak? Are they on fire, or do you wait until the flames die off?
I have the PizzaQue for my kettle. I use pieces of oak barrel staves or chunks of my old oak tree over Kingsford Comp. A couple 2"x6" pieces will last less than 10 minutes, or one pie. I can get it up over 700 degrees, but the oak pieces need to be flaming for that to happen. After the flames die off, the temp drops dramatically. Hopefully I'm done with the pizza by then. When I'm ready to do another, I throw on a couple more oak pieces a minute or so before sliding in the next pizza.
I only put the fuel around the back side of the kettle, not much under the stone, if any, and none toward the front.
I'm still working out the perfect situation, but feel I'm pretty close here.