Abit LG-81 random shutdown/heat issue?

Greetings Everyone,

I'm trying to build a new machine, and I've run into some issues
here. I grabbed an Abit LG-81 mainboard from an online retailer. I
attached an Intel 640 3.2 GHz chip, with a Thermaltake Golden Orb II on
it. I equipped this unit with 1GB of PQI Dual DDR Kit (5400) RAM, a
Western Digital 320GB hard drive, and recycled a Ben-Q DVD-RAM drive.
I've installed a MGE 400W Switching ATX power supply, with 2 fans on
the PSU.

Problem is, after it is all assembled, and setup, OS is installed,
and software (Office, Creative Suite, Various players, burning
programs, etc.) is all setup, the machine is exhibiting an issue. Mind
you, this didn't start until this machine had been running flawlessly
for more than 24 hours. After being booted, and running (mostly at
idle) for between 3 minutes and 2 hours, the machine will click off. It
does not shut down, it simply clicks off. At first I thought it was a
glitch, phenomenon. Then I began to try to reproduce it. Upon first try
with Prime95, I was able to make it click off. I initially ran the
torture test to see if it was a heat issue. I wanted a program to be
able to stress that. I loaded Everest Home Edition, and the FanEQ
program that came with the board. I was successful, in a way, to get
the machine to click off after about 15 minutes. Now when I say click
off, you can literally hear the power supply click, as it's going from
the on position to the off position. The fans all spin down, and leds
dim and fade away. Windows does not shutdown. Nothing ever gets logged
in the event logs. Seems to be hardware related.

I spent hours repeating this with mixed results. At one point I
enabled the fan alert noise, in the PC Health page in the bios setup. I
was actually able to make the machine sing (with this alert beep)
several times. During each of these tests, the machine appeared to be
normal. The fans would all be spinning, very well I might add. The CPU
temp would not exceed 38 degrees Celsius! I continued this testing, and
seemed to isolate this as a heat issue. Even while testing, when
periodically the temps would read 117 Degrees for a second or two, the
chip was never that hot. I could touch the bottom of the heatsink
without it being that hot. Not burning. NOT 117C. This seemed to
isolate a heat sensor issue, potentially. Even though these high temps
were observed, It was only twice, and they NEVER showed over 38 degrees
Celsius. I've sat and watched several times, observing temps realtime,
and they don't spike, or show above this range at all. The temps slowly
climb, then stop @ 38 when the fan's cooling reaches a temperature
stasis.

I tried to isolate this some more. I went back to the main
symptom. The switching power supply is being told to shut off. I wanted
to test this, just in case this was software related. It appeared that
even if this machine was at idle, it would turn off in an hour or two.
I proceeded to download memtest86+ from www.memtest.org. I booted the
machine from this disc, and allowed this to run for 26 hours. It made
almost 200 passes on this 1GB of ram (in two sticks) and found 0
errors.

Now, that's not the power supply. But it does show that the
machine can run for 26 hours outside of WindowsXP. I then decided to
try another PSU. I grabbed a 500W Antec Neo HE PSU, and installed this
in the case. I booted the machine, loaded Windows & Prime95, started
testing, and within 10 minutes the machine clicked off. I confirmed
this a few times. It's not an issue of the machine not getting enough
power. It's stable, and the voltages on both PSU's are reading
identical. Both in Windows, and on the Multimeter's screen!

Just for kicks, and out of frusteration, I rebooted, and allowed Xp to
load. I walked away. 24 hours later, still @ idle, the OS remains, and
the machine is still powered up. I've tried applications testing, and
mock "working" on the computer, and the error does not occur. In fact,
this error occured first @ idle.

So, now I'm left pulling my hair out. I'm left with no other
option as to why this is happening. I've disabled all warning/shutting
down for fan failure/sensor temperature. I've experimented with this,
and I've been unable to isolate this further.

BTW, latest BIOS (01-16-2006) and all of the WindowsXP updates are
installed. Please help me Abit community.

-John