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Parts for Frigidaire PLHS267ZCB2 Refrigerator - AppliancePartsPros.com

It is normal for the evaporator coils to frost up and then defrost.
If the freezer temperature is OK and the compressor runs/cycles normally then it sounds like a drain problem.

The drain tube is plugged so that the water from the defrost cycle cannot drain into the drain pan.

I would check that it is clear.
It is under the evaporator coils in the freezer and then drains down the tube (item 84 in section 13) to the drain pan under the unit.
If plugged a turkey baster with warm water, an air compressor with low pressure or a wire can often be used to clear it.

Also check that it is not plugged with dust etc. at the drain pan end.
You might as well clean the condenser/compressor fan and the condenser coils while you are down there.

If it looks like stuff is growing in the tube I would flush it with a 10% bleach solution. This should kill the stuff and let you flush it out. You may have to do this several times, let it sit for 15 minutes or so between flushes.

Be careful that the drip tray does not overflow and empty the drip tray after you are done.

Be sure to unplug the unit while doing this.

If frosting up is excessive then check your door seals.
Get a two inch wide strip of paper and place it between the frame and seal, then pull it out to check the seal.
Do this every 6 to 8 inches around both doors.
Even if the seal is bad the drain still sounds like it is plugged.