I've had 2 different techs check it. They and everyone they consulted with said thermostat. Replaced it-no change. When checked in morning the top is usually 0 and the lower bottom is 50 degrees.
The last guy said the top 2 right coils were not frosted and this could be a sealed system leak. But they also said it could be the adaptive defrost control.
The fridge is not overloaded, fan working fine. 30 degree air coming out of freezer vent into lower compartment, no bad seals. The fridge is 2 months out of the 1 year warranty. (Fortunately I have the doubling of the warranty feature on my AMEX purchase. )The techs don't seem to be able to have any test for the defrost board other than replace it and see what happens. I want to avoid throwing new parts in as a test without knowing. If it's a sealed system problem is it even worth fixing? Any more ideas and info are much appreciated.

Thanks for the response. Another day another Sears tech. He pulled the back of the freezer panel. The frost pattern was fine. Took a photo-evenly coated. He changed the evap fan motor PN# 2315539 when the phone-consulted head tech said there was a better fan. He had the part with him. Exactly the same as the old part-not an upgrade. Installed it-no change.
After 24 hours bottom of fridge is 52 degrees. The top has been variable with zero being achieved but lately seems to be 20. He knew no other test to do. Could it be the defrost system?
Of course its on a chip. Any ideas? Thanks

There are dual returns- both clear. The gasket is good. Will check the bulb door switch. Plenty of air coming through the vent from the freezer before and after the fan motor change. The top is only 20 though. Might that be not enough to
keep the bottom in the upper thirties where it should be?