I have a 5 year old Whirlpool refrigerator with top-mount freezer. Several time over the last month I've taken items out of the freezer only to find them partially thawed. I put a temperature logger in the freezer for several days and see that the freezer chills to ~0°F and then warms up to ~20°F. The warm ups don't appear to be the defrost cycle. Those appear to be occurring as well (based on sharp sudden spikes in the chart). This is a more gradual, several hour increase. I've troubleshooted a number of things and am not certain on my next step. Let me run through what I've checked so far:

1. Checked freezer coils during operation - didn't seem like there was excessive frost, but may have been based on when in the temperature curve I had pulled the cover off.
2. Defrost thermostat - checked continuity under warm and cold conditions. Appears to be working properly.
3. Defrost heater - Continuity check seemed to indicate it was working. Also, brief sharp spikes in the temperature profile seem to indicate this is working as well.
4. Fan in freezer - operates normally
5. Coils under the fridge - These were a mess. I vacuumed and cleaned as best I could. They're not very accessible. Look much better, but certainly not like new. This cleaning appeared to make no difference in operation.
6. Freezer temp control flap - Not broken or stuck. It seems to me, based on anecdotal evidence only, that turning the freezer colder makes the problem worse. In the least, it doesn't make it any better.
7. Compressor and fan - The fan runs when the compressor is running. The compressor turns on and doesn't make any odd noises when running. That being said, it seems to me like the compressor goes for very long periods of time before turning back on. I've found that turning the fridge to a colder setting, and then putting it back to it's normal setting (3) kicks the compressor on. Then the refrigerator operates normally for a few hours until the refrigerator reaches temperatures, the compressor kicks off, and then sits idle for a long period of time allowing everything to warm up.
8. Freezer door seals - As best I can tell, these are in good shape and performing normally.

I'm at a bit of a loss as to what could be the problem. The only two things I can think may be the problem is that the refrigerator thermostat is sticking/failing or the ADC control board is messed up. I've reset the board several times, but I'm not seeing any real improvement (maybe briefly, but it's hard to tell). I'm thinking it's not the compressor or refrigerant being low since it's able to cool fine when it's actually running.

Any suggestions? My next step is to just buy a bunch of parts and start swapping them out in hopes one of them is the culprit.

Thanks, denman! I was leaning toward this as this morning I had tried another experiment. In previous cases, briefly turning the thermostat cooler kicked the compressor on. This morning, when I observed melted items in my freezer, I turned the refrigerator thermostat WARMER and the compressor kicked on. This further bolstered my belief that the thermostat was sticking and not telling the compressor that it was too warm in the refrigerator. I'm going to order the replacement part and see how it responds. Thanks again!