My guess is that just having that one vent open allowed enough air in to your charcoal bed to let your temps get away from you. I've used lump for years in the 18.5" WSM and it's usually not a problem holding temps where I need them.
I know we have all been taught that you should never close down the upper dome vent, but don't be afraid to just to close it down to 1/4 open as opposed to wide open. Creosote development doesn't happen as easily when charcoal is used as a fuel compared to wood (I no evidence to prove this, it's just my opinion).
There is a video that Rock BBQ has put out that shows how to load a 22" WSM for use with a stocker. In the video they use lump, and they recommend closing the upper dome vent to 1/4 open for easier temp control. The video also has some great information on using lump in a WSM:
YouTube - Rock's Barbque Fire and Cooker Setup
Direct link to the Rock's BarBQue Page:
Shelter me from the powder and the finger
Cover me with the thought that pulled the trigger
Think of me as one you'd never figured
Would fade away so young With so much left undone
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