GE Refrigerator Cooling problems

Question

My "inherited" GBX25 Refrigerator is well cared for and in almost new condition. However it only gets "cold" in the freezer and "cool" in the fresh food area, but ice cream melts and canned drinks are slightly cool. this has been a persistant problem in spite of 2 repairs. Everyone is just guessing at the solution. I set the thermostat to "coldest" in the freezer and there was no difference. The compressor was replaced about 3 years ago. I clean the coils regularly and the fan seems to run fine. The whole thing just doesn't get cold enough. Is there a "defrost" issue here? Can it be a faulty thermostat? Also what is the small 'one-way' plastic adjustment s***w behind the thermostat panel located between the freezer dial and the fresh food k**b? I can't find the schematic for this model. It is under a plastic plug and not accesible without removing the whole cover panel.
Thanks,
Sam