First off what kind of incubator are you using? Second what kind of fan? Third a proportional versus non proportional stat does make a difference in temp spikes. I have a walk in incubator which is 6ft wide by 5 ft deep by 7 ft tall give or take. I only jave one small computer fan running constant. Now the way i have it set up is the fan is at the top blowing down at the space heater. Im using a johnsons non proportional stat and my temps stay at 88 to 89 degrees. If your fan is at the bottom it may be causing your spikes depending on size of incubator. Another thing could be the fan motor irself could be generating heat. But i doubt it. Still a possibillity. Alittle more info on your incubator would be helpful.