The machine is messing with you and is going to be a pain to find the problem part.

Since you have already replaced the defrost timer and the thermostat, I would be inclined to replace the heater perhaps it is intermittent. This is uncommon but not unheard of.

I cannot find any tech info on the web about this model so am assuming that the timer is the mechanical type.
You may want to mark the cam and the machine's frame /cover so you can tell where the cam is. Then check it periodically to ensure that it is turning and not hanging up somewhere in it's cycle. There is always a possibility that you received a defective new timer.

Usually a defrost cycle should occur once every 6 to 8 hours for 20 to 30 minutes, the compressor, it's fan and the evaporator fan should all turn off during defrost.

Would also re-check the wiring. Make sure the plugs and receptacles are clean and fit snugly and are crimped properly. Visually inspect and run your fingers along the wires sometimes they can break internally. You may be able to find a spot where the wire looks/feels deformed, could have a small crinkle or a larger or smaller diameter, Pay special attention to where the wires make turns.