I would suggest ditching the charcoal as your primary fuel source and start using splits. Starting the fire with a chimney of coal with a split or two is a great way to get a quick bed of coals but that should be the end of the charcoal for the day. Offsets are designed for stick burning unlike the Eggs or WSMs and by maintaining a small hot wood fire you'll have a much easier time of holding temps. Offsets require good airflow and if you put too much charcoal into the firebox at once you always run the risk of too much of it igniting at once and then you end up with runaway or wildly fluctuating temps. Start with a chimney or less of charcoal and a large split or two smaller ones, as that burns to coals add another split and so on. The idea is to have the wood combust and not smolder as soon as it is put in the firebox. Combusting wood makes very little smoke which is the thin blue stuff that you want. You'll be putting a split on every 45 min. or so, your exhaust damper should be wide open and your intake should not have to be choked way down to control the heat, (mine is usually about a third to half of the way open). Just try this method and adjust as needed since all cookers are different but I think that it will really help you to get control over your offset.