While the fix from June last for several weeks, the intermittent problems have cropped up again. After finally making in in before the Library opened this morning, the technician was able to diagnose the problem: overwork!
Our elevator is not designed for the constant use it receives. With over 500,000 visits last year, that is a lot of trips up and down, especially since you cannot get a stroller into the Children's Department without it. Apparently, the constant use causes the oil to overheat which cause the shut-down. I guess that answers the "why does it always happen when we're busy?" question.
On to the next fix: retrofitting with a cooling system for the oil. Our biggest roadblock is that it would need to be vented outside the building. Let's hope they can find a clear (physically, as in no pipes in the wall, etc.) and simple means of doing so.