The diagnosis from the dealership is that the oxygen sensor on the van needs replacing. This is not too surprising, and fortunately shouldn't be excessively expensive, either; maybe less than $200. I think it probably explains a fairly significant drop in mileage as I drove north, too. They could have it ready today, but I told them that Friday would be fine, as we have a part on order (a new sounder for the "you left your lights on" buzzer) that isn't supposed to arrive until tomorrow anyway.
They can't get the AC fan to fail, which is also not surprising, give the intermittent nature of the problem.