The problem that you are having is the defrost timer is not working properly. The condensor coils in the freezer side are iceing up, and when they do the fan can no longer push air acrost them to cool the freezer. This is a common ailement for a unit as old as yours is.
This can be changed by a person that is mechanicly inclined. I do recommend that you have a technician do the repair as there are a few things that are good to have checked at this time in a refrigerators life. The freon and oil level in particular.
The service call and the part will run you about $100.00 to $150.00 depending on the area you live in. After that your unit should give you many years of good service.
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