Refrigerator cycling on and off does not maintain temperature

This began imperceptibly, and increased in frequency over several months.  Now the temperature controls cycle on/off many times a minute.  The compressor runs.  I have read that this is not the computer control board from another post on a different website, where replacing that board did not fix the problem.  Is it a defrost switch problem? This is a model KSSC42FMS03.