Sounds like you have a restriction in the system. Try this: Unplug the refrigerator and leave it for at least 24 hours (48 hours is better). This works maybe about 20% of the time. Plug the machine back in. If the restriction isn't too bad, this sometimes will clear it up and disperse the comtamination. If it doesn't lead to an even frost coating on the evaporator (if you get the same condition you started with),you'll either need to call in a pro to repair it, or look for a replacement refrigerator.

After unplugging for 36 hours, no ice build up on coils, but refrigerator or freezer is not cooling