Freezer cools properly. Compressor stops and won't turn back on.

I have an older Liebherr closed system fridge/freezer CS 1640.

The fridge portion works fine but the freezer can't maintain it's temperature.

While the unit is pretty old now, since the fridge still works well I want to see if I can save it from the scrap yard by fixing the freezer.

Initially I kept finding ice forming at the bottom of the freezer floor. I'd chip it off and just close the door again, but the same scenario kept occuring. Eventually I opened the back panel to find the drain frozen over.

Defrosted everything and let it dry out.

After a week or the freezer alarm triggered because the freezer temperature was falling. I found that the freezer compressor was not turning on.

I removed and checked the start relay and overload protector. They seemed ok.

After plugging back in the unit and turning on and off the power button the compressor turned back on.

However, after reaching the temperature set, whether I put it at -18 or all the way to -24C, the compressor will turn off and not restart again until some combination of turning it on and off a few times, running it first through the diagnostic test program then turning it off and on, or unplugging and re-plugging.

The diagnostic program didn't trigger any errors. All four temperature sensors in the fridge and freezer return readings that are pretty close to the ones I get off a temperature gun.

I also pulled out the main PCB and visually inspected for any blown capacitors or fuses. There was no signs of any damage anywhere. Although there is some part that rattles when you shake it. It's hard to tell which part has causes that sound though.

Evaporator fan runs.

I can keep resetting the freezer once it warms back up and getting the compressor to turn on and get very cold but obviously this is an annoyance. Other than getting and maintaining ice, I wouldn't be comfortable putting food back in.

So I'm pretty stumped if there is anything else left to do.

Liebherr doesn't even make parts for this unit any more.

Just seeing if anybody has any experience with a situation like this and found a solution.

Any suggestions appreciated.