I think it is your stat. I had the same problem when I made my first bator, a hot water stat just doesn't cut it.
https://www.backyardchickens.com/a/homemade-incubator-4
As I write this post I am in the middle of my new build, a mini frig bator that will hold a 40 egg turner, and I am using a STC 1000, PID controller, http://www.amazon.com/gp/product/B008KVCPH2/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1
with a CPU fan to move the air. I will post some pics and facts as I get it done.
Good Luck, Phil
That to is what I use for my homemade food heater bator. But I have noticed it gets stuck and errors but it I tap on it it will start working again