You gotta love a mistery
I can tell you that over the years, I have
expierenced these type of "gremlins" on
several occasions on all types of hardware.
In one case, we expierenced unknown crashes
about once every 30 or 40 days.
I just started pulling each board
one by one to examine them closely and I found
one with a deep gash across several of the onnector pins at the card edge. I actually
dabbed a bit of solder on each scratched
connector and the problem was never expierenced
again.
When ever I see these unexplained crashes, I
just shut the machine down and go into it and
reseat all the boards, memory and connectors
and the results are more often that the problem
never surfaces again. That's not to say you
should skip running diags. though. Running them
can point you in the right direction if not
actually find the problem.

Good hunting