With a coking coal, the air has to be right on to get proper burn - yes, proper burn still means that you will get coke trees - but the idea is to adjust the air so the coke burns up w/ out building too high and make sure that the coke trees are not "hard" coke. Hard coke is what coke plants make and this type of coke occurs with coking and agglommerating coals under certain conditions in underfeed stokers - too much or too little air. Hard coke is dense and does not like to burn - it's also more uniform in appearance, you're not able to see individual lumps of coal stuck together, but, rather, a solid tower or spires w/ cracks.
Changes in coal or quality can be a real PITA; it's too bad they're not more consistant - they're probably processing whoever gives them the best price on coal and not keeping to a narrow range of properties in they're requirments.
Having said all that I said in the previous post, probably the best thing you can do to prevent outfires (esp. w/ a coking coal) is to leave loose ash in the furnace. In mild weather, I often have loose ash an inch or so above the door before I bother to remove it. In cold weather, I only remove clinker. Stir the loose ash around the retort w/ the poker after you removed clinker. You don't want the ash to pack tight. The loose ash should not be removed and should be about the level of the fire door (in very cold weather with a large heating load, the loose ash level will often go down because the hot, prolonged fire will clinker much of it, but, that's not a big deal in those conditions).
Also, you may want to further reduce the feed rate on the stoker, you want longer run times and longer cycles. I use a pulley w/ an 1-1/4" belt diameter on the motor.