On Aug 1, 3:35 pm, Darryl wrote:
I've never seen anything quite like this, which is why I'm beginning
to suspect the motherboard. Only the MB could produce such a
devilishly odd problem. If it was thepower supply, I would expect
the problem to be less subtle or more intermittent.

Nothing performed says anything is good. Remember, it is not a
binary world. Everything in that computer is definitively good,
definitively bad, or unknown. After all that work, everything is
still unknown.

Start with the one subsystem that can make everything else appear
defective. That means a 3.5 digit multimeter - a tool so standard and
so simple as to be sold even to K-mart shoppers. You need one. Also
Available in Radio Shack, Lowes, Wal-mart, etc because the tool is
that useful.

Use this two minute procedure to obtain voltage numbers on wires
between motherboard and power supply. Remember, power supply
subsystem is much more than just a power supply. The procedure in
"When
your computer dies without warning....." starting 6 Feb 2007 in the
newsgroup alt.windows-xp at:
http://tinyurl.com/yvf9vh
Pictures for assistance with above procedure;
http://techrepublic.com.com/5102-10586-5566528.html
http://www.helpwithpcs.com/courses/p...nc-pinouts.htm

Post those numbers from each wire before power switch is pressed and
when power switch is pressed. It may identify a problem immediately
AND those numbers posted here mean better informed replies can be
posted.

That computer could have been defective two months ago. A defective
power supply can still boot and operate a computer. Only way to
confirm 'definitively good' is numbers. Get the meter. Use the
procedure. Post results. Once a power supply subsystem is
'definitively good' then move on to other informative procedures that
have not yet been performed.