Samsung RS21FCSV (problem)

Hi ~ I’ve just come across this excellent forum after typing in “Samsung Refrigerator Problem” in Google.
I’ve just read some of the other threads & now realise I am not alone with the problems that have just developed with our fridge freezer after nearly four years of good service (apart from a fragile water dispenser lever).
Symptoms: first noticed that the 2 degree fridge indicator was flashing on and off and that the fridge wasn’t as cold as usual ~ the lower area of the fridge appeared colder than the upper area though. I adjusted fridge temperature and each fridge number flashed.
Decided to turn it off for 15 minutes assuming this may “reset” the fridge.
When i turned it back on the fridge reading was 16 degrees which seemed more like reality. The fridge reading stayed at 16 for about two hours and then gradually counted down to 7 degrees and then slowly counted back up to 18 where it has stayed (thereabouts) ever since.
The freezer, water dispenser and ice maker are all working fine (touch wood) and there are no noisy motor sounds or signs of water collecting at the bottom of the fridge of frosting up (as experienced in other threads).
The two top button diagnosis check reveals the bottom horizontal bar on the second fridge temperature digit.
I am a quite handy and hope to be able to carry out a simple repair if instructed ~ I would appreciate any help with the diagnosis and an idea of the parts that I may need (and approx. cost).
Many thanks for anyone who could take the time to offer advice.