UPDATED log of what's been done
This is a SUMMARY of the problem, and the steps I have taken so far.
THE PROBLEM:
At apparently random intervals into playing some 3D games the monitor goes black. In a couple of seconds, the green light next to the power switch starts flashing (like it's in standby mode).
Occasionally a sound effect will loop 2-3 times before going silent, as if the system were hanging.
The system fans keep running, and the MoBo lights stay on.
The numlock key will no longer toggle the numlock light afterward (on stays on, off stays off).
Restarting the monitor does nothing - the light goes green, then returns to flashing mode.
Rebooting the computer with the reset key brings the system back normally.
Note that I am not overclocking, never have overclocked, and never intend to overclock.
WHEN IT OCCURS:
Playing 3D accellerated games, at apparently random intervals. It can happen in intense graphics moments, or simply at the start-up screen. Shortest duration a game was up was about 45 seconds, longest was about 25 minutes. It can happen if the game is left idle as well.
Games when it has occurred:
Pirates of the Caribbean
Mechwarrior IV Vengeance
Medieval Total War (a game that ran fine on my 7 year old Voodoo 2 system)
Various 3D demos of games
BREED (FPS+ vehicles) demo
Tachyon (Space fighter sim) demo
Armored Fist 3 (tank sim) demo
Comanche 4 (helicopter sim) demo
Games when it has NOT occurred:*
*At least not during the hour or more of time I played.
Stronghold and Stronghold Crusader (2D)
Command and Conquer Tiberian Sun (2D)
Homeworld: Cataclysm (3D)
IL2 Sturmovik (3D)
It NEVER occurs during non-gaming computer use, internet use, etc.
Some demo games's missions are short enough that they can be played through entirely, closing out the game normally, thereby ruling out any given file or process as the cause.
My system:
ASUS A8N-SLI Deluxe motherboard
--SLI set to single card mode
AMD Athalon 64 3800+ (512 cache)
1xNVIDIA GeForce 6600GT GPU, 128 MB DDR3, PCI express, SLI capable (not using), DVI
2x 1 GB RAM, DDR400, ULTRA brand
--2GB total
ULTRA 500w power unit
Seagate 120 GB SATA hard drive
BENQ CD/DVD R/W
Recycled my Gateway Plug&Play CRT monitor (VX900)
--It is 7 years old but has not given me problems ever
Windows XP Home Edition, service pack 2
WHAT I HAVE TRIED (that has not worked):
*Changed / lowered refresh rates, graphics quality, color quality, and screen resolution
*Upgraded all WinXP patches
*Changed NVIDIA driver to ver. 66.93 - newer versions (70.90) are reported as buggy.
*Upgraded ASUS A8N-SLI deluxe BIOS to 1011 (final)
*Various game patches installed
*Removed all extra drives except primary (E:, a SATA hard disk)
*Used prime95 to stress test the MoBo, RAM and CPU. 45 mins of smooth testing, none failed, system temps (CPU, RAM, MoBo) remained below 52c.
*Turned off ASUS Cool'n'Quiet utility - reported in online forums as causing system shutdowns. Later uninstalled it.
*Turned off all power management settings in Windows
*Checked the BIOS automatic power-down settings - all set well above the values I have observed just prior to shutdown.
TEMPERATURE TESTING:
*Reworked inner cables for better airflow - all round cables
*Installed extra fans (2 front, 2 rear front-to-rear airflow), as well as CPU fan, chipset fan, side panel fan, GPU fan, and powersource fan.
*Monitored GPU temps during regular computing and 3D gameplay. Temp values do not rise outside of tolerance ranges. However, the NVIDIA monitor does not leave a log of temps behind, so I have had to alt-tab out of games at intervals to record manually the temps - meaning if there is a sudden spike I could not observe it.
3D gaming: highest recorded GPU temp 53c, CPU stabilizes at 50-52c (measured by ASUS probe, which leaves a log).
(note: I have a Big Typhoon large CPU cooling fan ready to install, but I am using the OEM packaged CPU fan atm. Since the temps are not implicating a problem, I don't think installing the BT will help this issue.)
PROBLEMS THAT MIGHT BE RELATED:
On initial install, Windows XP found four items of hardware that it tried to find drivers for - but failed. At the time we were just trying to get basic computer operations working, so I had XP ignore those.
Since this problem cropped up I went back to find them in Hardware Manager, they are all listed as unknown, and three of them map onto the XFX NVIDIA graphics card - even though the graphics card itself is recognized and installed. I do not understand this - is Windows not properly understanding components of my graphics card? THe fourth item is something associated with setting up RAID systems, which I am not using.
Also The mobo chipset fan suggests that its max speed is 6600 rpm, it regularly runs at 8000+. The mobo temps are not out of range, but this fan seems to be running very fast for some reason.