Thermostat like hydrofarm allow way too much fluctuation, I do not trust them with my snakes and would definitely not trust them with my eggs.

Good RELIABLE thermostats Herpstat, Vivarium Electronics, Helix

One more thing not sure what you are using to monitor your temps but keep in mind that the most accurate way to do so is with a temp gun.

If you have a digital thermometer with a probe inside your egg box it can be 2 degrees off or more, so getting a second reading with a gun will let you know where your temps really are.