One of the biggest mistakes people make is to keep chasing temperature fluctuations. The air temperature will fluctuate but the egg temperature will not fluctuate for every air temperature fluctuation. Also, make small adjustments and give them time to equalize in the incubator.
Imagine you are heating a room with a baseboard heater. The temp closest to the heater will be highest while temps further away from the heater are lower. You need to let the air circulate in the room rather than to keep adjusting the knob higher as the heater cycles off and lower as the heater cycles on. The air will mix to become somewhat constant but it will continue to have warmer spikes right next to the heater everytime the heat comes on to compensate for warm air cooling in the room.
If you put a glass of water in the room and let it get to room temperature, it is not going to be as hot as your hottest spot or as cold as your coldest spot and the temperature of the water will not go up and down each time the heat comes on and kicks off, changing the air temperature in certain spots.
The place you want to measure your temperature is in the egg zone, not above or below. The air above the eggs, closest to the heating element, will be warmer than the air below the eggs, farthest from the heating element. I put a probe thermometer right next to the top of my eggs and aim for 100 degrees since my thermometer does not measure in decimal increments.
As long as the temp in the egg zone is within a degree or two, the eggs will maintain the correct temperature range because they work as heat sinks. The more eggs in the incubator, the more steady the egg temperature will be because there is less air space.
Hens heat the eggs through direct contact with one side of the egg and they turn the eggs periodically to keep the cold side from getting too cold. We do the same thing warming in front of a fire and our body temperature does not fluctuate as we face the fire or face away from the fire. The egg turner helps the eggs keep a balanced heat but the important thing is that the embryo floating around in liquid inside the shell has a heat source that keeps its body temperature steady.
As long as your location for your incubator is not drafty with huge temperature swings, the incubator will even out the air temperature around the eggs and they will hold a somewhat constant temperature. Don't get too caught up with 10 second readings or you will overadjust the thermostat and not allow the air temperature to stabilize.
I was reading somewhere that you can measure egg temperature with an ear thermometer but that would require opening and closing the incubator and the air temperature would never stabilize that way. I may try it on hot days just to be sure my eggs are not cooking when the house heats up too much.
