freezer cold, fridge warming up

Hello alt.home.repair -- I'm trying to repair a KitchenAid refrigerator (freezer on the bottom).
I'm fairly certain the problem is the defrost system. Periodically the fridge compartment starts to warm up. Cold air comes from the vent in the freezer section, but little or no air reaches the fridge. It stays warm until I unplug it and let it sit for a day or two. Then it works fine for a few months.
The third time this happened, I pulled off the rear panel in the freezer section. As expected, it was very frosty. It looks like the air intake in the freezer section is eventually getting plugged. I assume this is why the fan is unable to blow much air up to the fridge, though it does circulate air in the freezer.
So... I've tried to isolate the broken part, but they all seem to be working, at least to the untrained eye. If I manually turn the timer, the heater comes on. If I then wait 20 or 30 minutes, the heater turns off and the cooling system starts. To see if the timer was initiating the defrost cycle I made a tiny wax peak and stood it up on a bit of aluminum foil next to the heater. Checking it about 10 hours later, the wax hadn't melted, however it was tipped over in a pool of frozen water in the bottom of the foil. So something melted the frost.
I'm a bit stumped now. Could one of the parts be working intermittently? If so, any idea how I would identify it? I could replace all three, the timer, heater, and thermostat, though it's a bit more money than I'd like to spend.
Alternatively, perhaps the kids are leaving the door open, and more frost builds up than the defrost cycle can remove. If the door is left open very occasionally (say once a month, for 30 or 40 minutes), should that be enough to overwhelm the defrost cycle?
Thanks.
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Hello As you may already know there are only 3 things that need to work correctly for it to defrost. The defrost thermostat, heaters & the timer. So if you start having ice buildup in the freezer, make sure the defrost timer is working. Most of them you can manually over ride (older units) as you found out. So if you turned it and the freezer started to defrost then you can say that the Heaters and the thermostat are ok for now. I would get something and mark the part that you turned on the defrost timer. Then check it after a few hours and make sure it's moving on it's own. My guess is that your defrost timer is bad (it's common).
Good luck, Jeff
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Just a WAG, the timer is running slow. The timer comes on at less frequent intervals. So it's not defrosting enough hours in a day / week. So, the frost is building up.
Could also be a slow evaporator fan. Not pushing enough air through the evaporator.
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The fan in the fridge has nothing to do with the defrosting stage and the fan for the compressor runs when it does. It sounds like you might need the thermostat (this will be mounted near the heaters) or check to see if the doors close like thy should. Like you said before maybe the door is staying open.
Just my thought.
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The thought about the evaporator fan would explain if the fridge was warm all the time. Like you say, should be off during defrost cycle.
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