What are you using to take temp readings with?
Thermistor senses 'fin temp' which is 10*F colder than 'box' temp.
Thermostat is pre-set and thermistor location should have a 6-8*F affect on overall temp in food section.
When you say 'thermistor has been replaced'----------was that a complete replacement or just the tip (snip the tip repair)?
Poor connection at lower board P2 connection can cause overcooling
Thermistor tip should just protrude from clip and clip should be tight on far right fin
And it could be issue with lower board
Any adjustment of thermistor position needs 24 hour before remeasuring of affect