First, I'm not an expert on thermostats. Your inquiry intrigued me so I did a little research. The problem may be in the bi-metallic thermostat. It does not "control" temperature as precisely as a proportional thermostat like the Herpstat brand. The temperature fluctuations you are seeing are normal for a bi-metallic thermostat...from what I can tell. Of course those temperature fluctuations are not appropriate for the needs of ball python eggs. I use a Herpstat on a newly set-up cooler (check my DIY incubator post) and the temp doesn't fluctuate more than 0.5 degrees Fahrenheit as measured by two thermometers placed in the incubator. I'd recommend a proportional thermostat like a Herpstat for more precise temperature control.
Matt
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