Whirpool fridge freezer 20ri-d4

We have a Whirpool 20ri-d4 American fridge freezer it is about 15 years old.Two days ago I noticed all the frozen food had gone soft & the temperature was running at plus 8 degrees which went upto 12 degrees.All the food had to be thrown away and after searching for a possible fault I found that it could be dirty coils so i took the cover off and they were covered in dog hair & dirt & fluf I suspect they had been like that for some time.I completely cleaned them & the temperature eventually came down to minus 20 (it may have been higher but my dig thermometer only goes as low as -20) the freezer temp settings were et to minus 18.The temperature starting getting warmer about 4am & ithought it must be a defrost cycle and eventually settled around plus 1 degree.I went back to bed expecting it to be -20 degrees again when I got up but it isnt its still around -1.
Is there anything else I can check as I am reluctant to call somone out becuase if it has packed up given its age I dont want to spend a lot of money on it & will have to think about buying a new one which I didnt want to do as its a nice well built fridge freezer but at the same time dont want to throw good after bad