This is my first time to try to hatch chicks. When I called the representative he asked if the black wire had a piece of plastic stapled to the end. It didn't. The end was resting on the screen. I guess that was the problem. I've monitored it now for over 24 hours and the temperature has held steady.
Ok, now that makes sense! Mine has a thin clear plastic square attached to the end of the sensor. This allows it to ride on top of the eggs as they turn, which would be about an inch above the egg turner.