GE Profile Arctica-Fridge works fine, freezer too warm

I have a GE Profile Arctica, side by side refrigerator. It is 6-7 years old.

I have tried everything to fix the problem with this fridge. It has worked fine since new in 2006 but about 3 weeks ago the freezer and fridge both started getting way too warm, seemingly overnight, as if we left a door open. I removed the back cover and discovered the condenser screen was totally clogged with dust, like it had a blanket over it. Cleaned all dust away from the screen. Did a test of mother board and discovered a burned out resister for the evap. Replaced resister. I also replaced the overload relay and the PTCR switch on the side of the compressor. Now the fridge part works perfectly and temperature is right in the safe zone where it should be but the Freezer side, no matter what I do - hovers around 20+ degrees and never gets any colder. There is frost on the back cover in the freezer unit and then it seems to go through a defrost cycle and 24 hours later there is no frost but the freezer - no matter what - never gets colder than 20 degrees. The frost on the back comes and goes so it appears that the defrost mode is working. I can hear the fan in the freezer unit working when the door is shut. A Freon loss seems like a possible culprit but certainly doesn’t explain why the fridge side works perfectly. Any suggestions would be appreciated