Hmm.. I didn't think about the humidity possibly causing drips onto the sensor. That would cool it off quite a bit. When I look at what the sock does to the humidity thermometer - it reads 85 when the regular thermometer reads 99 because of evaporation cooling at 30% humidity - imagine it would be much worse at 60%. I would guess it is causing the probe to mis-read the temperature and turn the heat back on - causing the spike. So - moving the probe or putting it in something that does NOT allow humidity to drip on it would be a very good idea.
I love the depth you have available to put those nice little individual hatching boxes in... !!! very nice!