Refrigerator not cooling-freezer section icing up

Refrigerator not cooling, ice forming on back of side-by-side freezer:
Turn off unit. Empty freezer section. Remove freezer back panel (4 screws) to expose iced-over coils. Use a fan to speed up defrosting. Put a towel or a plastic drip pan in the bottom of the freezer to collect melting water (rather than letting the water collect and maybe overflowing in the drip pan underneath the refrigerator).
After defrosting, by temporarily replacing the freezer back panel and turning on the unit, the refrigerator may operate OK for about 3 weeks before the next excessive ice build up.
Possible problem- a burned out Defrost Heater.
To check: After defrosting, turn off power to or unplug the refrigerator.
Unscrew the Defrost Heater bracket (2 screws). When loose, unplug the two wires connected to the unit. Have the unit checked, or use an ohm-meter to check continuity. If unit measures "open", high resistance, replace the Defrost Heater and bracket. If the heating element shows continuity (or low resistance) the heater is probably OK and you have another problem. Now you may need to contact a repair man.