I have a GE fridge freezer, model number TFG28PFDA WW, which is 20+ years old. For a long time water was building up and freezing in the bottom of the freezer. I worked out this was a problem with the defrost drain pipe and cleared it, so that’s no longer an issue. Very occasionally I get the 'de' error message, but I don't know why - there is no ice and no frost.

But for a couple of years the temperature control seems to have gone awry. In the summer the temperature in the freezer has gone up to -10 degrees Centigrade, and in the fridge to +10, and in the winter the freezer has gone down to -21 and the fridge to -1. They should be -18 and +5 respectively. It makes no difference whether the 2 temperature slider controls (both in the fridge compartment) are set to 1 (least cold) or 9 (coldest).

The compressor is running almost continuously, only stopping occasionally for a few minutes, and some days not at all. And it often runs very unevenly - speeding up then slowing down in little bursts.

When I dealt with the defrost pipe blockage, I cleaned where I could, underneath and at the back, and I noticed that the compressor fan was only turning very slowly. That’s the only visible fault that I am aware of. I also disabled the ice maker as it was rarely used. The water is off and the switch arm in the ice compartment is in the up position.

I've found a lot online about fridges OR freezers being too warm, but very little about one or other being too cold, And nothing addressing this problem of both fridge AND freezer being both too warm or too cold - depending on the time of year and hence the ambient temperature in the room. So from my description above, can anyone suggest the likely cause?

I appreciate that any part(s) required may no longer be available, but I need to be absolutely sure that it's beyond repair before considering junking and replacing such a large and expensive appliance.

Thanks.