For the last 2 months my freezer icing up.After removing back panel of freezer there is build up ice on coil.I tested Heater, Evaporator with meter and works ok.Then i replaced thermostat and having same problem.I try few times defrosted using hair dryer but after 5 days its icy again.Looks like fridge works and works.Everything in my freexer is so frozen and hard.Anybody knows what can be the problem?

thanks Tom