Just noticed that you were cooking on a Chargriller, I have the same one and have cooked many, many butts on it but have since moved to an UDS and a home built smoker.
I had problems keeping the temp consistent at first also and used a ton of charcoal along the way.
Here are a couple of things I learned with mine:
Keep it out of the wind. Seems like the slightest change in wind direction can really make the temps go crazy, both up and down. Ive done everything to stacking hay bales around it, moving to the other side of the house, to using a big cardboard refrigerator box as a wind break. Just be careful not to catch everything on fire, lol.
Keep your charcoal piled up rather than spread out over the whole firebox grate. I used a couple of bricks in the firebox to keep the charcoal from spreading out. I also started building a small fire inside the cooking chamber opposite the firebox, just 15 - 20 briquettes to start and add 3 -4 every hour or so. I eventually made a charoal basket for the firebox but kept using the bricks inside the cooking chamber to deflect direct heat. You can still cook 4 butts at once this way, two on the upper shelf and two on the bottom.
I think these two things really helped me to start turning out some pretty darn good barbeque. Hope it helps. Just remember, practice, practice, practice.