If the probe is inside the egg box, then any temperature variance will be harder to correct as the flexwatt has to keep heating until the inside of the box reaches temperatures, then the flexwatt stops, but the outside of the egg box is a higher temp and then it cycles cool. This causes swings in temperatures that take a long time to settle down until everything is stabilized. Add to that the heat from the fan and it can cycle even more. I have a large incubator (upright freezer) and my probe is right in the middle of the incubator, not in an egg box.

Also, I would look at the Herpstat documentation and see if it has a tolerance zone that can be adjusted. I have older Herpstats, so I can not help with this setting. Good luck.