PC DL Deluxe - After three years, system won't boot and gives "System failed due to CPU overclocking"

So I'm about to toss my very expensive computer off the roof and hoping
someone can help. In 2003 I built my dream system as per the
following:

Asus PC DL Deluxe
Dual Xeon 2.66Ghz
2GB Mushkin RAM

I do not and have never overclocked this system. Everything is at
stock timings and has been the whole time. For three years, the system
has been rock solid. I never get bluescreens and run MemTest86
regularly. Zero errors.

All of a sudden, I am having serious problems getting this system to
boot up. Once it's up and running, everything is great. No errors, no
bugs. But if I have to reboot, I'm in for hours of headache. 9 out of
10 times on power-on I get POST beep and then nothing. If I watch
close, I can see the monitor blink the splash screen for a fraction of
a second and then go dark. If I sit there and continue alternating
between the reset switch, the front power button and the back (PSU)
power switch, it will catch and then boot up.

Earlier today I flashed the BIOS from 1007 to 1009 to see if that
helped. While it did not resolve the problem, it did change the
behavior. Now whenever it fails to boot up, the voice system comes on
and says "System failed due to CPU overclocking". Twice since then,
I've gotten it to power up and go past POST. It gets all the way up to
the Windows XP splash screen and then blacks out and gives the same
"System failed due to CPU overclocking" voice message.

So... Can anyone help??? I have passed the point of frustration with
this thing. Since it's worked for so long and then stopped, my first
instinct is that a piece of hardware died. I find it hard to believe
that it's the proc or RAM because everything works fine once it gets
booted up. I have it running MemTest86 right now with zero errors.
Part of me hopes that it's the PSU because that will be the easiest and
cheapest to replace, but I don't have any reason to believe that is the
case. When I am able to get into the BIOS settings and check Hardware
Monitor, the voltages all appear fine. So, that leaves what is
probably the problem... Dead/faulty mobo. Unfortunately, the PC DL
Deluxe is not readily available anymore or I would just switch it out
and see what happens.

Does anyone have any thoughts? Something else I can try or look at?
Do you think it really is a dead motherboard? If so, can you recommend
a source for a replacement? I can't find anything but eBay and I'm
leery of dropping $225 with no chance of return.

Thanks in advance!

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