A few other random thoughts...
If you send something back, put a small mark somewhere with a Sharpie; that way you can tell if you got a new or refurb instead of a repair.
To differentiate new from refurb, or repaired from no problem found, look the PCB(s) over carefully for a little shiny residue (rosin). In production that is cleaned off but in rework it is rarely removed.
For this particular problem which seems to be temperature and humidity related, and infant mortality, I would guess you have an active component (all of which will be encapsulated in non hermetic epoxy cut with silica with no RTV prior to encapsulation) with a bad wire bond. You'd be shocked how cheaply non milspec electronics are made, and still work pretty well. Why they eliminated the other PCBs from being returned must reflect some experience they have with the problem, or cost containment.
I got a replacement PCB for my T5z from Taylor earlier this year and spotted the rework rosin on all three pots. Since it was free and worked, I did not complain :~).
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