Freezer too warm - thermostat or leak?
Sounds like you're low on refrigerant. The sealed loop internal pressure has dropped low enough that all the evaporation is happening in the top two layers of the evaporator coil.

You could replace the refrigerant, but the question will remain: what caused the loss? I suggest that a qualified ($$, I know) repairman is your best chance at finding and repairing the leak.