I put all of the charcoal to one side and put the roast on the other so I was not over direct heat.
I did have to play with the vents to keep my temps consistent.
I found that I the small vent openings on the cast iron top were not enough.
I slid it almost 2/3 of the way off to keep my temps up.
Still, I was thrilled with the outcome!
Might want to keep an eye on the holes in the bottom grate. They can become clogged with ash, which will inhibit airflow to the coals. That might be what is causing the temp issues.
Most of us have fashioned or purchased a "wiggle rod", which is just a thin metal rod bent at a 90 degree angle, which you can stick inside the bottom vent and use to dislodge the ash from the holes.