I have been having a devil of a time getting the temperature on this incubator set right. I was using it this spring and it was a diehard, set the temp and it held it well. Now, it is being a royal pain. Temperature is too low, I edge it up a little, still a little low, edge it up a bit more, and then it's jumps a degree. It will stay between 98 and 100.5, but 99.5, or even consistently close is being very elusive. Is this a bad thermostat? Dirty thermostat? Thank you!