[gjbingham]

Junglepizza,
Did you or your builders use any insulation below the floor of the oven? I believe the traditional mud ovens did not use insulation under the floor, so even if you got the thing to 800 degrees, it would loose its heat to the surrounding substructure.

If there was no insulation put under the floor, I'd either add an additional layer of firebrick on the floor - in essence, build up to increase the heat retaining abilities, or work on a technique with the fire to surround your pizza with the coals on all sides - as close as possible. Perhaps even raking the coals back across the floor for 5 -10 minutes after each pizza, then clearing and brushing it again just before the next pizza. Its a big oven so it may just take a ton of wood to get the thing hot enough to retain the heat necessary to keep the floor hot.

A picture might just solve the whole question. The chimney set-up sound unusual. You might be loosing all your heat out the pipe.