[COLOR="Blue"]When the defrost cycle starts it runs for a couple days and totally melts everything in the freezer before the cold cycle starts again and everything refreezes.[/COLOR]
The defrost thermostat cannot do the above.
The way it works is that the defrost time is a fixed time usually around 30 minutes. but lets say that the frost is melted off the evaporator coils in 20 minutes, then the defrost thermostat opens. This kills power to the defrost heater and the defrost cycle continues for another 10 minutes but the heater is off..

Sounds like the defrost timer is sticking in a defrost cycle.
I would replace it.

I am assuming that the unit is inside and not in a cold area like a garage etc.

From your description of the frost on the coils this may not be a defrost problem.

Is the compressor running?
You can usually hear it but also should be able to feel it vibrating.

Does it run all the time other than when the unit goes through a defrost cycle?

This unit should go into a defrost cycle every 8 hours of compressor run time and it should last for 30 minutes.

If it is running all the time, remove the cover in the freezer and check the evaporator coils they should have a fairly even coating of frost on them.
If not then it is probably a sealed system problem.
This would be an expensive repair and often it makes more sense to put the dollars towards a new unit.

Make sure that the compressor has run at least an hour before doing this check. If you do it shortly after or during a defrost cycle then you could end up with the wrong conclusion.

Below is a link, take a look at the 3 example pictures about a third of the way down the page.
http://www.applianceaid.com/ref...ator-not-cold-enough.php