I have a Whirlpool top freezer model # ET1MTMXKQ00.

About once or twice a day, it will almost completely thaw, then freeze solid again. The refrigerator compartment doesn't seem as cool as it should, also.

I was thinking it might be the defrost thermostat?

I had the same problem with my freezer 6 months back, over a period of about three weeks it kept defrosting and refreezing again, but each time it refroze the temperature kept dropping, till eventually it defrosted for good.

I called the hotline and the repairman came and said that the gas had leaked out, he replaced the copper piping and refilled it with gas, and 6 months later its perfect.