KitchenAid Built-in, Model KSSS42qdx05, Manuf. date 1998 (21 years old)

I'm trying to avoid replacing this unit and hoping someone out there might be able to help.

The refrigerator side remains about the proper temperature all the time. The freezer cycles through warming up to the mid-30s, then back down to the single digits. It stays in the low single-digits long enough to make some ice. But it stays in the mid-30s long enough for ice to melt. Same thing happens when it is empty or full of frozen food.

- Interior freezer fan is running properly
- Condenser is clean, condenser fan is running
- Compressor draws the proper current when it is running and cooling (around 1.6amps)
- Compressor draws about .38 amps when it allows the freezer to go into the mid-30s.
- Compressor draws nothing when the defrost cycle is running
- It does make a sort of gurgling noise when the compressor is running, before and after being topped off with 134a.
- UV flashlight and 134a detector were used to try to find leaks - none found
- When the compressor is properly cooling down the freezer to single digits, there is frost on most of the evaporator, it seems to be working properly
- The defrost timer was recently replaced, the fan and compressor turn off entirely when the defrost timer kicks in, the defrost warming coil warms up properly, the defrost thermostat seems to be working correctly (proven by the fact that the compressor kicks back on)
- The position of the freezer thermostat knob doesn't seem to affect the temp. I can turn the freezer and fan off with the knob, but otherwise, where the knob is turned doesn't seem to have any affect on the temperature. I will admit the feedback loop is long and I haven't been extremely precise in my measurements. If it only makes a +/- 5 degree difference I probably wouldn't have noticed.
- The freezer thermostat has been replaced with a new one.
- The compressor overload circuit seems to work properly. If I power off the whole unit while the compressor is running, then turn it back on right away, the overload circuit kicks in and only allows the compressor to start up again after the pressure in the system balances out and allows the compressor to start up. The compressor tries to draw about 14 amps if it is turned off and then right back on again. That's when the overload protection circuit kicks in and turns off the compressor.
- The evaporator pan under the freezer is dry, there doesn't seem to be any corrosion or issues with the heat exchanger under the unit.
- The capacitor seems to be working, otherwise, the compressor wouldn't come on at all (I think).
- I topped off the r143a several times, as recent as about a week ago and it's been able to cool the freezer to low single-digits with the compressor running at the proper current draw (based on the service sheet) for several cycles of cold then warm, etc.
- Pressure on the high side (according to manifold gauges) is higher than the specification sheet indicates it should - pressure is around 300 when it should be less than 200 ziga-somethings (sorry, not exactly sure what the units are, PSI?)

2 service technicians came out and diagnosed that there is a leak in the case that can't be fixed. But I can't believe that is what's happening because it can get the freezer down to low single digits. Any sane technician probably sees this as a complete waste of time, with a low probability of success, so that would be a reasonable answer. But I am not willing to give up since there is a $11K purchase at the finish line if I throw in the towel.

If it were a leak or other major issue, it doesn't seem like it could operate properly some of the time. But, I can't tell what else might be wrong and I'm hoping someone in the forum has encountered behavior like this in the past.

Bottom line - fridge is cool enough all the time, freezer cools itself to low single-digits for long enough to create a couple batches of ice (maybe 12-15 hours), then warms up to mid-30s (long enough to melt the ice it had made while it was cold). Then repeats the cycle.

Any help will be greatly appreciated. It's a puzzle and I need some help to try to solve it.

Thanks!