How are you building your fire? I start with a basket of unlit charcoal & a few chunks. I leave an open spot opposite the side vent to add lit charcoal.
I fire up about half a chimney
I have the stack wide open & the side vent wide open. I add the lit charcoal to the basket.
I leave the side vent wide open. If I'm shooting for a cooking temp of 240, I'll close the side vent down to only about 1/4 open when the cooking chamber temps hit about 210-215. This will slow the temp rise & allow them to stabilize. When the temps stabilizes, you can put your meat on. You will get a temp drop when you open the chamber to put the meat on. Allow 15 mins or so for the chamber to get back up to temp. If you are above or below your desired temp, you should be able to dial it in with small adjustments to the side vent. I normally get at least 2.5+ hours of stable temps with this method.
When the temps begin to drop, I will rake the lit coals to the corner of the basket & reload it with unlit charcoal & a few more chunks.