I have a little giant incubator with a fan installed and a temp/hygrometer inside too, but I have had fluctuations down to 97 degree. How can I keep the temp stay normal and increase my hatch rate of duck eggs?

I would bet that the room temperature is fluctuating which will cause the incubator temp to fluctuate. You need to keep the room that it is in a constant temp, then mess with the bator to maintain  the 100 degrees. Placing it by a window with the sun shining on it will raise it to deadly temps also. I will actually run my air conditioner to keep my room temp constant. Hope this helps.Also make sure your thermometer is calibrated. I place 2 or 3 in just to be safe. Make sure they read the same.