When using a accurite digital thermometer with probe, you will probably notice more temp fluctuation with the probe temp reading than with the main body of the thermometer. There are a couple of reason for this. One the sensor for the main body of the thermometer is located inside the thermometer caseing. Since the air doesnt circulate thru this caseing verywell, the temp reading will be slower to adjust to the proper temp. The sensor on the wire is more open and is also a different sensor than the one inside the main thermometer. With my accurite, the sensor on the wire will change temperature readings much faster than the main thermometer. Air movement will effect the reading of the sensor as well. Fast moveing air produces a cooler temperature reading. My suggestion is to take all of your thermometers and place side by side, including any probes or external sensors, and without any direct air current moveing across them. Give them time to stabilize to the current temp and then compare the readings. Any thermometers with probes that are off more than .2* between internal and external probe should not be used unless you note that temp difference and compensate for it. If your external probe is changeing temp reading at a much faster pace than the internal sensor while in the incubator, this is probably due to to much air flow across the sensor. Either move the sensor probe to another spot inside the incubator, or partialy block the air flow across the sensor. A piece of cardboard or other material can be placed between the sensor and the direction of air flow and will result in a more stable temperature reading. As the heating element turns on and off, you can expect a little overshoot and undershoot of the desire temp setting, but it is the adverage temp reading you need to be concerned about. Unless of course the undershoot and oveshoot has a range of several degrees.
