Close that top vent some when you're getting close to the temp you want to maintain. If it's all the way open, you're allowing the coals to run the show as opposed to inhibiting the drafting that it wants to do.
On any kamodo cooker, which are usually even more airtight, that's an important variable, and the principle is the same. (on my char-griller, I can leave the bottom open just a sliver, and control my temp between 220 and 260 adjusting only the top vent. When I need more than that, I usually just slightly nudge the bottom.