Domestic Appliance Forum

I have been using an old fridge freezer in the garage during lockdown but have been also using it to store homebrew pressure barrels following fermentation so I set up a temperature logger in the fridge and freezer sections just out of interest really to ensure that the temperatures are as they should be.
Perhaps this has always happened but I have not known until setting up the logger.
Please see attached extract of temperature log. There are two regular patterns that emerge - one very quick warming spike that mostly happens every 15 hours or so but then it cools down very quickly again. I presume this is the defrost heater kicking in which I guess is normal?
The problem areas are the longer warming period that are in between these times where the freezer gradually wamrs up over a 6 hours period. Clearly there's something not quite right - this suggests that the freezer has just switched off completely and started wamring due to ambient temperature.
I'm not an expert in freezers but I know the basic as of the thermostat operation, just not sure how the defrost heaters work really? Any help would be appreciated.