Make sure your incubator is in a draft free place (closet, closed garage, windowless bathroom, etc) where it does not catch sunlight part of the time. Make sure you are not opening and closing the incubator a lot. If the walls of the incubator are thin or styrofoam, it is probably sensitive to environmental changes (like how your house warms up a little when the sun is out, vs. at night).
A good suggestion is to add some form of heat sink- a few smooth, clean stones, a bottle of water, baby food jars with holes punched in the tops, a heat/cold pack like you'd use in a cooler to keep beverages cool. All of these things will take in heat when there's too much and give off stored heat when there's too little, helping to maintain a steady temp in the incubator.
If your humidity is fluxing a lot as well, that may be affecting your temperature slightly.