Fridgedaire Defrost Problem

Hello- New member here. I have searched the forums and have found some great advice and direction as to my fridge problem but wanted to see if anyone could help further. I have a 3 month old very basic top freezer style Fridgedaire refrigerator. I have tracked it down to a defrost problem. Same old ice build up on the evaporator coils and ice blocks the airway to fridge and doesnt cool. This has done this from day 1 since I got the thing. I have to defrost it manually every 2-3 weeks. I had a repair guy come out and he replaced the bi-metal switch and the defrost timer (supposedly). It is still freezing up. I think that the heater must be bad in it. I am going to test it, along with the timer (which is supposed to be new) My question is, could it be something else besides for the timer, bi-metal, or heating element? Like I said the thing is next to new, and I would not think that these parts are bad from the factory. I need to get this fixed and unfortunately, I have to fix it myself. It is not my fridge (rent) and it is more of a hassle to get my landlord to get it fixed than just fix it myself and I am not home during the days to let anyone in. Also the fridge is so new that no aftermarket has parts for it yet. Do I have to go straight to Fridgedaire? ANY help is appreciated. THANKS!