The second issue (the one I started investigating) is that (with the
cpu freq at 3401, enabling turbo mode, and I assume, actual frequencies
up to 5500MHz) the temperatures recorded start creeping upwards (when
the system is mostly idle, and nothing is really changing at all) and
what's more, that seems to be on an exponential curve (positive feedback
perhaps). That is, going from (reported values of) mid 30's to around 40
as the "resting" state, can take many hours, then from 40 to 50 or so, less
time, and then once it gets beyond 50 and is approaching 60, it might just
be minutes until it reaches Tjmax and powerd (or the cpu itself perhaps)
decides to shut things down (when powerd does it, I sometimes see its
broadcast message - but I often don't have a login terminal visible, so
often not) and once or twice, X has actually shut down, and I've seen at
least some of the normal shutdown sequence happening on the console.
Usually however, the power is (or seems to be) simply abruptly cut, and
everything simply stops, instantly, working and doing things (like typing
an e-mail, or whatever) one second, and no power the next. (And no, it
is not an external power issue, the system has a UPS, and in any case if
it lost external power, it would reboot as soon as that returned, this does
not do that, it behaves just like "poweroff" but seemingly without the
file system unmounting, ... that would normally happen.)