I agree with Chris A, it sounds like a Thermometer issue. Don't feel bad, I bought TWO thermometers through the mail and they are both WAY off. I probably should have returned them, but instead I used a cheap candy thermometer that was much more accurate to figure out where the needle on my newly mounted permanent thermometer should be. I still use it for reference because it's really easy to see (even from upstairs bedroom at night!). Someday I'll replace it, but not right away. I know when it reads 200 it's really 250 so it doesn't really matter to me.
One thing I would check is how much charcoal you are using, and if it is fully lit when you started your smoke. When I use the "Minion Method" with lump I usually get a good half a chimney going and then use the vents to keep it at around 230 - 250. Last weekend the thing just cooked away with lump charcoal without touching it for 13 hours!!! It's a really well designed unit, unfortunately not all thermometers are created equal.
Good luck,
-Rich