I have been using a good quality lump (http://www.nakedwhiz.com/lumpdatabase/lumpbag111.htm)
And lighting it with Harry Soo's donut method. And closing down the vents to hit my targets pretty close. I have been filling the water pan about half way and not had to refill. Despite the success early in the cook (hitting initial targets), I am having real hard time with big fluctuations - as much as 30+ degrees. The end product has been pretty good but I have really had to work the vents. I usually close all bottom vents most of the way after hitting the target. I have tried closing all but one bottom vent completely but the coals don't burn evenly in the bowl. So I keep all open the same amount (usually very little once hitting initial temps). The darn thing just won't hang in for more than an hour or so without adjustment. Very frustrating on overnight or long cooks.
Any advice from you lump / WSM users out there would be really helpful. How do you all get your WSM to sit tight for hours?
Thanks (and sorry for the long post!)!