If your smoker is throwing lots of thick white smoke it probably is burning your fuel at too low a temp. Also, you could have low quality charcoal (I've never used Stubbs so I don't know). As well, if it was rainy and the wind was gusting you may have had wind blowing down your chimney.
You mentioned that your smoke became thin but you would get some belching of thick white smoke. You may have had wind blowing down your chimney and when you got a little break in the wind the hot air was coming out the chimney properly. This can cause big fluctuations in your temp on your chimney side. If you think this description sounds like your day, you want to position your smoker so the wind is blowing perpendicular to it (ie, the wind is blowing west, you have your two ends, chimney and firebox air valve, going north/south). If you can't move your smoker you can make a wind block. Sometimes I see guys making one out of aluminum foil, kinda like a bent tube so the exit is facing away from the wind.
LG BGE-Brinkman Cimmaron-Custom Pull-behind (old grade school boiler)-Weber Kettle-DCS 30"