I aborted my firing last night after the kiln stalled for more than 3 hours at ^3/6 (side-to-side unevenness; that part was not unusual. It evens out later.) It hasn't done that for more than two years, since I corrected the height of the bag wall!
Needless to say I was grumpy but I am guessing weather conditions are the cause: Here in Central Maine we were experiencing sustained high winds almost steadily for the full duration of the firing. My thinking is that the wind created such a strong draw - on a kiln that is already inclined to draw too hard - that the hot gasses were just being sucked out of the kiln before they could heat things up. I will re-commence firing on Wednesday, and we shall see what we shall see.
While I was looking for some online confirmation - someone else who had had a similar experience - I discovered this really great discussion by Vince Pitelka on firing. It seems to be about a specific kiln at the Appalachian Center for Craft, but much of it is applicable to firing any reduction kiln. If anyone is curious about reduction or any other gas firing, I just wanted to direct them to this great resource - and to say again thank you to the giants among us who are willing to share.