Frigidaire Frost=Proof Is Icing

Question

I have a frigidaire Frost Proof top freezer refrigerator. The freezer is icing up on the back wall and I am not getting cool air in the bottom part. I have used the hair dryer to defrost and then we seem to work OK for a few hours. The fan in the freezer is working.

Model is FPCI21TFW1 s/n LA02406477. Tried to locate a manual online but I haven't had any luck.

Any help would be appreciated.