Ariston OS-KVE160 overcooling

Good afternoon Hope someone here can help as I am getting severe grief over the frozen lettuce!
Problem started in the very hot weather last summer when the motor wouldn’t cut out and I suspected that the heat was not getting away from around the built in cabinet. Also I replaced the thermostat with the recommended one from, I think, Ariston spares. Eventually that started to overcool so I changed it again to a cheap generic one. It seemed ok at first but often the temp would be down to -5 first thing in the morning.
I have read in the advice about KT59’s and OV’s or something but dont know which I have. The unit is a flat back wall with condensation or frost on it so I guess its a wet wall. The only other thing is that fixing screw that holds the sensor end back to the wall was very corroded so I have stuck the flat retaining plate containing the sensor to the back wall with aluminium tape- it seems to be making good thermal contact but I am now at my wits end. Surely we dont need to replace the whole fridge? 🙁
Sorry its such a long posting

Mike