Mysterious hardlock

Hi,

I've been plagued by a mysterious, unpredictable hard-locking condition for
about two months now. It is OS neutral--even attacking while FreeDOS is
idling! (I've tested with Linux 2.4 & 2.6, FreeBSD 5.2.1, FreeDOS, and
whatever memtest86 is) It doesn't seem to be related to any particular
memory stick, or the whether the sound card or graphics are is
initialized... I can compile the Linux kernel, or make world with FreeBSD,
so my system *seems* ok. A friend of mine lent me a copy of Eurosoft's
PC-Check. All the tests show green. I suspect a power-supply issue.

Does anyone have any idea how to debug power issues? (not necessarily PSU)
Does this sound like dirty A/C? Has anyone had any experience with
brownouts?

--
Sten