Turns out the problem was with the cache voltage as you suggested, cekim.
Final voltages are:
Vcore: 1.03v (though AiSuite reports this rising to 1.04v while stress testing)
Input Voltage: 1.8v (I believe this is stock?)
Cache Voltage: 0.98v (anyone know what the stock voltage is for this one?)
Do these voltages all seem OK, again with the focus being on longevity? (Hence the conservative overclock). Temps are fine.
Thanks again for any help.
Just a comment on the eventual voltage. My understanding is that you should only need to set the memory voltage on the main page and leave the eventual voltage alone if you plan to run at the same memory voltage as boot. When eventual voltage is set to Auto, the memory voltage during operation will match the boot voltage.
The reason for the ability to manually set the eventual voltage is for cases where you need extra voltage to get a successful boot but you can run stable at a lower memory voltage once successfully booted. The eventual voltage manual setting provides the ability to specify a LOWER memory voltage than the boot memory voltage upon successful completion of boot.