I have the same problem with my Piglet when I burn briquettes. Try using lump charcoal. The briquettes are denser than lump and take longer to burn. By the time the bricks you lit to start it burn off, the entire basket of charcoal is at a full burn, and by then it is hard to control. Lump does burn faster, but is easier to control by starting with one of your intakes completely closed. This causes the pit to come up to temp a little slower, but it is easier to control. I always leave my exhaust vents 100% open, and control the temp by adjusting the open intake vent.
Red Valley BBQ ~ KCBS CBJ ~ Backwoods Piglet ~ 270 Smoker ~ Traeger Lil Tex ~ 3-22" Weber Kettles