No BSOD, no logged event

Okay, so I've been having this ongoing problem that I can't seem to fix.

The Problem:
When running high-demand games that play online, such as WoW, and the more recent Dead Island, my computer crashes by turning off the monitor and sitting idle until I force a restart by pressing and holding the Power Button.

I used to get the crashes rarely while playing WoW, but they have become more frequent.

Also, it has crashed outside of games; recently it had crashed a few times while trying to stream video from places like Youtube. It had crashed both with only one window open, and one window with multiple tabs.
After a few restarts combined with some routine Maintenance, I have yet to repeat the crash.

However, I can repeat the crash with any demanding online game.
I can play other demanding games, such as Dead Space 2, Rage, and so on without ever causing a fatal crash.

I have:
Checked for Virus' and Spyware and have eliminated all that were encountered.
Defrag
Scan Disk
HDD Scan/Repair
Ran HijackThis and eliminated any suspicious Drivers
Cleaned the Registry using CCleaner
Thoroughly Cleaned the Computer physically and removed all Dust from the Fans and Fins
Updated all Hardware Drivers including MOBO-Software such as Chipset Drivers and Video Card Drivers
MemTest86+: 2 Passes with no Errors (However due to recent reading, I have decided to try this again to include 7 Passes and testing each Stick separately)
System Restore: This changed absolutely nothing, even when Restoring to the original Windows XP Installation day)

I have turned off the Auto-Restart to allow the Blue Screen Of Death, but never encounter this as my Monitor always turns off with no possible way to turn it on without Restarting.

I have also checked the Event Viewer and have not found any Red Tags.
A few Yellow Tags that pop-up:
NapAgent Event ID: 39 (Before this one ever showed I used to get a Red Tag "DCOM Event ID: 10016" but having followed some steps on the Microsoft website, it has since turned into this Error)
Dhcp Event ID: 1009
Dhcp Event ID: 1003
Dhcp Event ID: 1003


Months back I had encountered a severe crash that caused my computer to lock-up at the Windows Logo or Memory Test screen upon start-up. Having manipulated the RAM into different slots it has never occurred again, however I have also since moved the RAM back to the original slots and it has worked normally for six months or more.

I suspect it may be my RAM and/or PSU.
Any help would be much appreciated. Thanks.


System Specs:
XP Pro w/SP3
Gateway GT5220
CPU: AMD Athlon(tm) 64 X2 Dual Core Processor 3800+, MMX, 3DNow
Ram: 2048 RAM
Video Card: NVidia GeForce 210 1024MB
Sound Card: Envy24 Family Audio (WDM)
PSU: 300Watt (I'm unsure of this, I just wrote this because that is the stock PSU declared)
Internet: Logitech Wireless Card 54MB
 
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