I'm not sure the time the fire sits over the bricks matters as much as the time the whole oven is exposed to heat. I think James' previous description of building a fire that radiates heat and pushes the heat across the floor is relevant here. Especially if you have a big fire, if the floor has not been saturated with heat you end up with an imbalance of heat - yours was too hot over (although you really cannot be too hut but..) and not hot enough under. When the oven has more time to equilibrate this problem should be better. I had this happen even with a white hot dome last week, I think the white hot dome only means the dome is white hot, but the hearth bricks also need to be saturated with heat instead of just surface hot. This would be tough to measure, I think it's a feel thing. The other factor that I think I've run into is becoming better at prepping more pizza at once and keeping them moving in and out of the oven. It becomes a little harder to keep the floor hot because the pizza absorb some of that radiant heat from the fire.
It probably varies from one oven to another, but I wonder if there is a certain length of time anyone has kept their hearth at the 750 range to ensure enough time to cook multiple pizzas.
Can I suggest a covered area in front of the oven (perhaps all the way from you porch to the oven) to avoid future take out pizza?
