Dave,

Look at the freezer back wall,

If there is frost, snow or ice on the back wall you most likely have a defrost problem.

Even so, remove the freezer back wall and visually inspect the frost on the evaporator coils, it should be a nice even coat of frost on the whole assembly, no vacant spots or heavy build up in one area.

If all OK,

From there you will need to remove the control panel assembly from the ceiling in the refrigerator compartment,

Check the styrofoam block in the damper assembly for ice and debris, in the air direction tunnels all the way through to the exit port blowing the air on the temperature control capillary tube on the side of the panel.

Also check that the temperature mass barrel is lined up and in the proper position in the holder on the side of the panel, and the air is being directed onto the barrel.

If all the above checks are proper, suspect a bad temperature control.

Davey,

Bad News, based on your description of frost on the evaporator coil assembly.

You have a Restriction in the refrigerant lines,

Unless you're a refrigerator or A/C technician, you wont have the special tools and refrigerant,or meters to repair your refrigerator.

I imagine it would be a pretty expensive repair, you may want to call a local servicer and get a rough estimate of what it will cost you, first.

Then determine which way you want to go, new unit or repair.

Sorry for the bad news,

Good Luck,
:) :) :)