I had a Hoover same problem, it was under maintenance contract, and most times I would de-frost first before repair guy arrived, one time I didn't he looked at is and said your insulation has failed, uneconomical to repair and was scrapped, it seems ice forms inside the insulation and the ice is a good conductor compared with air, so switch off for few days and then back on and it seems A1, but as the ice builds up in the insulation the problem returns.
Test is using an energy meter, if for example it is listed as using 365 KWh/annum it clearly used 1 kWh per day, so if in a day it used 1.2 kWh then OK, but if it uses 2 kWh clearly a fault, the fault could be loss of refrigerant, temp set too low, doors opened too often, or insulation damage, but if not using too much power then you know insulation is OK and it is some thing else.