Have you modified the ash pan? Mine leaked air there badly. It's hard to get a long slow burn when you can't control airflow. I run with the damper 3/4 closed to maintain 275 and nearly 100% for 220 ish. Now that I modified the ash pan I can control it much better. It's been my experience that it's hard to control large amounts of lump because it burns so hot. Most drum smoker folks use briquettes for this reason. Load it up and let er smoke. Only problem is ash buildup and a couple kicks to the drum and its back to normal. I currently am using lump only to start my fire or for a quick temp boost if I nod off and the fire dies down.
Instead I have been using mini logs I get from the local Cabelas. 12 bucks for a 50 lb bag of oak or hickory and 16 for mesquite. Can't beat the price unless u cut your own and you cant beat a real wood fire for smoking. If you want to cook overnight while you sleep you need a gas or electric smoker. Tending the fire is part of the job when using wood or charcoal in a SFB smoker.