If I was reading right your thermostat probe is attached to the bottom of the uth and not sandwiched between the uth and bottom of the enclosure correct? My house is always 60-65 and a problem I was having with the thermostat probe being on the bottom of the uth is it would cause an temp fluctuations bc heat was hitting one side of probe and cool air the other. I moved the probe so it was sandwiched between the uth and bottom of enclosures and it fixed my problem.
Also you should always take your temp under paper towel or any substrate.
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