I have bought several different types of temperature thermostats TRYING to find one that will hold 99.5 degrees. Even the most expensive one varies from 95 degrees to 102 degrees. 80 percent of the time it will show 99 degrees. There is no such thing as one that will hold or even show 99.5 degrees. Is this swing of temperatures ok? How do you keep the eggs at 99.5 degrees?