Sounds like there is a defrost problem for sure. You won't be able to test much if you let it get warm. Plug it in, let it get down to temp. Ensure the condenser fan (behind unit by compressor) and evaporator fan (by the coils that are freezing up in back of freezer) are both running. Unplug the unit after you have it all apart and check ohms on heater and defrost t-stat. T-stat should be closed. If they check out ok and it can be forced into deforst, it needs a control board. On a side note, I have seen one of these where the freezer door switch went bad and was screwing with the adaptive defrost. Might want to confirm the switch is ok.