Something like this will also allow you to get a better sense of what's going on in the eggs rather than measuring air temperature.
http://www.ebay.com/itm/Egg-O-Meter-EGG-Incubator-Hatching-Thermometer-Bird-/271088192998?pt=LH_DefaultDomain_0&var=&hash=item3f1e1d8de6
All thermometers have different response times so that may be one issue you're dealing with.
As the heat element cycles on and off it will cause the different thermometers to read differently especially if one is closer to instantaneous.
Hygrometers IMO are notoriously inaccurate and need to...