I’m not sure knowing which model Hovabator or kind of controls would help in this one though sometimes you run into people that have faced a similar problem where that kind of knowledge might help. Can you describe the temperature fluctuation? How much and how often? Are there any particular times you notice this? Any tiny clue to your problem might be helpful.
How much do you trust your thermometer? Never trust any thermometer that comes with the incubator. Always get a thermometer you can trust so you know what you are talking about. It really helps to calibrate any thermometer that you get.
Where is the incubator located? Is it where an air vent blows on it, sunlight coming through a window can heat it up, or probably the worst, near an outside door so the temperature where it is rapidly changes a bunch?
Are you allowing enough time for the temperature to stabilize after you add water, open it, or do something else to it?
If it is a forced air model it should not matter, but in a still air it is very important where you take the temperature. Hot air rises so the higher in the incubator you take the temperature the warmer it will be. If it is a still air, are you taking the temperature at the same spot each time?
That’s about all I can come up with. If none of these seem to be the problem, I’d call the manufacturer, GQF, and talk to them. You may have a faulty incubator.
When I first got my Genesis Hovabator 1588 I had the same problem, but I first set it up in my attached garage so every time the garage door was opened the outside temperature around it changed dramatically. I moved it to an interior spare bedroom out of sunlight and away from any air vents. It holds temperature rock steady.