WSM and trouble with temps...

That's a tough one alright. Obviously you're an experienced WSM cooker since you know to keep it out of the wind . . .

Once I experienced strange temperature fluctuations with my WSM it was due to a different charcoal than I had been using. But other than obvious changes like that - it's difficult to tell where the problem may be. It may be the water pan or the fact that you have it loaded up more than normal. Those are good thesis propositions . . . but without accurate measurements of all variables and controls it will be difficult to say . . .

I've used stubbs with good results before. Did you shake your charcoal to make sure that it was tightly packed in the ring? I've done several cooks with 4 butts in a WSM, and not had a problem
When using water in the pan, it is a good idea to start with hot water, so that you are not wasting fuel heating it.
I hope this helps.

...or try using an EMPTY water pan.
I stopping filling the pan early last year and haven't had a problem since.
Now, I simply foil the top of the pan to make clean-up ez.

Sounds just like a bag of Stubbs that I used a couple of months ago.

Assuming you used two bags, did both do the same thing? I've used a lot of Stubbs in the last year and a half with my 18.5" wsm, and I've only had one bag do like this, and I noticed a lot more ash than usual as well.

Hopefully, the Stubbs will be more consistant in the future, but since Lowes didn't run the "two for one" sale around here, I might just order more of the RO chef's select (big briqs) through Doitbest.com. I did a test cook yesterday and got over a twelve hour burn with breezy cool weather with no windbreak, and water in the pan. Temps were very steady, 250 or so at the top vent for about eight hours or so before I finally needed to knock the ash off the coals by tapping the cooker legs with a rubber maul. These RO briqs burn a little slower than the Stubbs, but probably don't have much more ash, so I'm pretty impressed with 'em, at least for smoking.