Lump will flare up in the drum when it's starved for air so do not close down the vents unless you are shutting down for the night. I agree that when you opened the lid you created a back draft effect. Try using a blend of lump and briquet's with the heavy side going to the briquet's. Also you need to change or calibrate that thermometer.
Slappin that bass like some delerious funky preist!!!
UDS, Half Pint (Mini UDS), Weber, ProQ 20, Kegrilla
*250 gallon cooker- 'The Meat Beast!!'*
British BBQ Championships
Grand Champion 2008
British BBQ Society - Southern Championship
Grand Champions 2009