That is odd for a hovabator. Usually turning a full 360 degrees of knob only changes it 2F. Are you using only 1 thermometer? Using several or more will get confusing if all reading different. And if a still air it will have hot and cold spots, not to mention at differing elevations so need to take temp in same place.
Three things to note. If your home heating is being adjusted up and down it will effect your incubator. Set your house thermostat to a temperature and leave it. I put mine at 65F when incubating. Or if heating with wood find your most stable room temperature wise. Second thing is the eggs will provide a thermal mass to store the heat, keep wild temperature swings stable. Add a few bottles of water in lieu of eggs for now to hold steady heat. Lastly, check to see if the wafer itself is screwed on tightly, it may have come lose so getting wild readings as your adjusting the top dial it should be secure to.
With those kind of drastic changes with small adjustments it really makes me think the wafer is lose.