I have seen this kind of issues everywhere. I have tried everything I know and everything I found on the web for several months and I can't figure out what's the problem.
I have assembled my rig several months ago and from the very beginning I experienced this annoying issue.
Motherboard: MSI P67A-G43 (B3)
CPU: Core i5 2500
GPU: XFX RADEON 6950 1GB
PSU: Corsair TX-750 V2 Enthusiast Series
RAM: 2 x Patriot G series Sector 5 PC3-12800 1600MHz 4gb ddr3
HDD: Western Digital 2TB Caviar Green
So, the issue I have is that my PC freezes whenever it pleases and it forces me to do a hard reboot.
Most of the time it happens when I'm browsing the web with Mozilla, with IE or Chrome is much less frequently.
When I'm gaming its totally fine except for a few exceptions:
1) Playing COD Black Ops it freezes at random intervals, sometimes after 5 min, sometimes after 10 min, or more prolonged periods of time.
2) Playing UT2004 is fine. But playing using the Ballistic Weapons mod it causes the same issue as in Black Ops.
3) Playing COD World at War is a weird thing. Solo play is fine but when I'm waiting in the LAN lobby it happens the same as in Black Ops.
Things I tried
1) Reinstalling Windows
2) Tested my GPU using FurMark for about 20min. It never froze, even the fan was at 100% and the temp were as high as 80°C.
3) Tested the RAM using memtest for about 3 hours resulting in 3 passes 0 errors. Also tested my RAM with the memory tester that comes with the mobo.
4) Replaced my mobo for a new identical one.
5) Installing and using a second PCI ethernet adapter.
6) Checked the RAM timings, settings and voltage. No matter what the values are is still the same. I'm currently using the default settings: 1.5v, 1066 mhz, XMP disabled.
7) Tested with just one stick of RAM.
8) Tested with the other stick of RAM.
9) Updated the BIOS with the latest update.
10) Cleaning the registry.
11) Updating all the drivers and Windows.
12) Checked the PSU voltage in the BIOS. It was fine.
13) Installing just the basic components: CPU, PSU, GPU, HDD
14) More things I don't remember right now.
1) Sometimes I can't reboot my PC using the reset button, its locked up too! Forcing me to press the power button for 5 seconds to reboot.
2) The screen goes into a random color when it freezes. Sometimes it's 2 colors (stripped thin vertical lines)
Please any suggestion is welcome. I am sick and tired of this bug and I want to get rid of it as soon as possible.
You've done everything correctly regarding troubleshooting the issue. At this point I would say that you have a poorly designed MB, and they all are going to act this way, so I suggest biting the bullet and buying a different MB.