I've got this refrigerator in my basement, I'd say it's about 25 years old. A few weeks ago I noticed that it wasn't cooling very well (40°F on the thermometer). My first thought was that the coils must be dirty, so I tipped it back and gave the coils a good cleaning with a brush and my shop vac. It seemed to help, which surprised me, because they weren't that dirty. After a few days I noticed that the temperature was high again, so I took a look at the wiring diagram and decided the problem was either the defrost timer or the thermostat. I ordered replacements for both and swapped out the timer first. That seemed to work, but again after a few days the temperature was elevated. I then replaced the thermostat and nothing has really changed, Sometimes I check it and it's 28°F and other times it's 40°F. There was nothing in the freezer and the refrigerator has a 24 pack of Coke cans in it. I've now put a gallon of water in the freezer. I'm continuing to check it but the temperature swings don't really make any sense to me. Any suggestions will be appreciated.

This panel is inside of the freezer compartment?