I noticed that I also have a prob keeping my temps down cause too many coals are lit (I use the minion method in those cute little baskets Weber makes). So, I take out a few (relative term, depends on how bad you are overshooting) hot coals, close it up, and when you see your temps go down, start opening up the bottom vent.
Had to do this yesterday during my cook. The kettle wanted to sit at 260 instead of 225. I normally just let it ride where it wants, but I didn't want to screw up *that* cook.