Problem with Thermostat K2649

I have assembled a k2649, and it reports correctly the temperature after i did the adjustement at freezing-point and boiling water.
I need the range temperature from 50 to 100 celsius and i have mounted a 33k for R2 and 12K for R33 (in the manual at page 17 there is a wrong table that indicate R2 and R3 but it’s R2 and R33).
What is not working is that when i push the S1 button to set the temperature the display shows 00,9 and moving RV1 changes this value from 00,1 to 0,15 but after few seconds the display returns to 00,9.
Even changing the R2 and R33 or soldering a trimmer the result remains the same.
Another strange thing is that without pushing the S1 button (the display shows the actual temperature) if i move the RV1, the display changes the actual temperature for about 2 degree.
Any idea ?