Where is the heat tape? I would put the probe ON the heat tape. If the heat tape is on the bottom, and the probe is at the top, the bottom is going to get a bit hotter by the time the heat rises to the top, so after it shuts off, it's going to continue to still raise to the top. If the probe is on the heat tape, it will shut on and off more frequently and take slightly longer to initially come up to temp, but it will never be allowed to get too hot. Placing some full, sealed water bottles in the incubator somewhere can help to stabilize the temp so that when you open the door to check eggs it doesn't take as long to reach the correct temps again.