never had a lump either. probably caused by too much powder creating a small avalanche.
when that happened to me, i blew the part of and re-coated the part.
i used an infrared thermometer at first and now i just use the oven on light to set the timer.
set temp to 450*, when the light goes off, turn temp down to 400*, set timer for 15 min, turn oven off, open door, remove when cool.