Fridge/Freezer problems

The minute a bit of hot weather gets here, the fridge packs up (I know its probably related!).
Quickly turn on the spare in the shed and then that packs up.
I have two queries, for convenience I'll put them in separate posts.
I have a tall Philips Whirlpool fridge/freezer. Unsure of model no. as I can't get to the back. It has done faithful service over the past 10-12+ years, and has never gone wrong other than me having to replace a temperature sensor.
Yesterday I suddenly realised that it wasn't working properly. The light was on when the fridge door was open and the compressor was chugging away, but the temperature was rising - I have a remote thermometer stuck in the fridge. This was affecting both the fridge and freezer.
I turned it off, and stuck a fan on the floor blowing towards the back of the fridge, assuming that the compressor or whatever was overheating. 30 mins later I turned it back on, but still the same. I then switched it off and left it off overnight. Switched back on this morning and it worked OK. This afternoon it started warming up again (compressor still running). Turned it off for 20 mins then on again and it now seems OK.
I have had a similar problem once or twice before when the fridge or freezer (especially) door has been left open for some while (usually overnight) and the fridge/freezer has been running continuously for a long period. I just assume that something overheats and although he compressor doesn't stop, it just impairs its working. I assume the same thing happened now because of the excess heat (30-35 degrees in the kitchen) and the fact that the fridge/freezer doors were continuously being opened for water and ice cubes.
Is overheating likely to be the cause of the problems? I know loss of refrigerant can also cause a similar problem, but this doesn't normally occur, only when it's running excessively/
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