Remember that the environment that the incubator is located in has a lot to do with it's stability. If it is in a room where the temperature fluctuates often then the incubator will have a hard time maintaining a constant temperature inside. If the incubator isn't completely full of eggs and you have the room inside the incubator you could add a bottle of ~99F temperature water to act as a stabilizing "heat sink". A fan is highly recommended and will help to keep hotspots from forming within the incubator.
But, with wide temperature swings I'd look first at the environment that the incubator is located in. Once you get the environment stabilized you should have most of the battle won. Then, if the incubator keeps fluctuating drastically you may very well have a faulty thermostat.
Just some thoughts,
Best wishes,
Ed