I have been working this issue for a while and tried everything I can think of.
The radiator eventually boils out all the water from the radiator causing the engine to not cool properly. Once I fill it back up it runs fine. It takes many weeks and possibly months for this to happen and I give up daily checking for a while and then before I know it the water is low and it's hot.
I have changed the thermostat. I had a diagnostic code that said the fan relay was bad, so I replaced that. I flushed the system. I replaced the draincock (because I broke it doing the flush) and that is tight and leak free as ever. The system doesn't lose water at rest when parked. No fluid lost while stopped. It has to be lost when it is running and on the road.
One thing I am wondering about is the A/C. It is cooler now in California so the A/C is mostly off (though the wife loves to have it on once it hits 70 degrees outside.) The last few days warmed up to above
80 and I know the A/C was turned on. Is there a place that could lose water in the A/C?