I have a 12 yo GE SxS that is not staying cool enough.

I had this problem back in July, as well. There was uniform heavy frost on the freezer coils. The defrost heaters were bad. I replaced the heaters, defrost thermostat, and defrost timer all at the same time. After that, the appliance worked fine.

A few days ago, I noticed that the fridge was not cold enough again. I keep a thermometer in there, which read 48 degrees. Freezer felt cold, but was not making ice.

I took the back panel off the freezer to look at the coils. The top coils had a light frost. The bottom coils were a block of ice, not frost. I defrosted it. I thought maybe the heaters weren't working properly and took them to an appliance parts store to be tested. They were fine.

I put everything back together and started it up again. For now, it is working fine--fridge is at 38 degrees. However, I left the panel off to observe the coils. I happened to open it when it was in defrost mode. The top coils were cleared, but the bottom coils only melted slightly and then refroze. If I can, I will try to attach the picture.

Can anyone give me any insight? Is this a compressor (I can hear it running) or a sealed system problem possibly? I am willing to buy a new fridge if necessary, but of course, would rather fix it if it was cost-effective. It's 12 years old, so maybe it isn't worth it.