Freezer cold, refrig warm

Unit is about 5 years old. Initially the refrig and freezer started to get warm and things in the freezer started to melt. When the compresser cycled on it would only run for a minute or so. I replaced the thermostat attached to the refrig rheostat. The compressor would then run and the freezer went down to almost zero degrees but the refrig would range between 43 degrees and 50 degrees. I opened the back of the freezer and everything was clear and dry, no frost buildup or clogged vents. There is a "freezer control" attached to the freezer rheostat which looks to me like it adjusts a flapper allowing more or less cold air to go from the freezer box to the refrig box. Could that be my problem or is there something else?
Thank You
Don50914