Random freezes, occasionally BSODs

Hi folks,

Problem: Random computer freezes, i.e. hard locks, requiring me to do a reset. These occur especially when playing 3D games, but very rarely even while only browsing. Once about every twenty freezes I get a BSOD instead of a freeze, telling me there is a DRIVER_FAULT with the Realtek HD Audio Driver.

I mostly play "Warcraft III TFT", but I had freezes with some rather new MMORPGs as well, such as "Age of Conan" or "Warhammer: Age of Reckoning".

My Computer:

• Mainboard: MSI P35 Neo2 FR
• CPU: Intel E8400 3.0GHz
• Memory: 2x2GB G.Skill DDR2-1000, running at 800MHz effectively
• PSU: be quiet! Dark Power Pro 550W
• HDs: 500GB Seagate with updated firmware, 250GB Samsung; both SATA
• Gfx: Leadtek 8800GTS 512MB
• Windows XP SP3
• mouse, keyboard, WLAN stick are connected via USB; no further USB connected devices

What I've already done:
Hardware changes:

• installed new hard disk: Seagate 500GB
• exchanged SATA cables
• detached DVD and floppy drive
• checked and renewed thermal grease thrice
• exchanged WLAN stick with a different one
• exchanged PSU; had a lower-quality PSU before
• exchanged graphics card with a friend's one; after the next crash, reinstalled my own again; I did the same thing with a friend's CPU and RAM. Hence, I had only one of the three exchanged at a time
• exchanged motherboard with another Neo2 FR

Yeah, I indeed exchanged almost every component hardware, even if not all at one time. Note I'm using my own CPU, memory and graphics card again, i.e. the ones listed above.

Software/BIOS related:

• repeatedly updated drivers, updated BIOS
• reinstalled Windows XP twice; had it installed on each hard disk
• disabled onboard sound in BIOS; freezes continued
• slightly underclocked CPU and RAM
• played around with memory voltage and clock
• deactivated anti-virus program

Tests:

• chkdsk
• tested RAM with memtest86+
• did some stress test with CPU and graphics card
• checked PSU voltages with a multimeter

Temperatures:

• CPU: 40-50°, in idle slightly below
• CPU cores: 50-60°, in idle slightly below
• memory, north- and southbridge passives as well as capacitors passive next to CPU are a bit more than hand-hot
• GPU: 60° in idle with fan at 37%
• hard disks: ~30°

More: The BIOS displays only 4.1V-4.3V for the 5V-SB voltage. Although I've exchanged PSU and mainboard, the voltage is still displayed that low.
I've already had opened the chassis and turned fans to full power, still having freezes.

I'm really am at my wits' end and don't know what to check/do next. I'd appreciate any help.

Thanks in advance.