Sudden and frequent hangs on desktop computer: mobo or CPU fault?

I have a desktop computer equipped with an ASUS Crosshair 2 Formula and a Phenom x6 3.2GHz CPU. My problem is that often the computer will hang all of a sudden, completely stopping responding. When that occurs, reset key is inoperative and power button turns the computer off but is unable to turn it back on. I have to physically disconnect power cable.

The problem can occur anytime, when I'm booting Windows, when I'm logging in, when I'm listening to a song, when I'm browsing Internet, etc. It always occurs after very few minutes of 3D gameplay

I thought it was a video card fault. I had 3 8800GTX so I could try all combinations of them: didn't fix

I thought it was a RAM problem: I tried running with only a subset of my DDR2 banks but didn't fix.

Almost every time I have to reset and reconfigure BIOS (without AHCI, Win7 won't boot, so I need to restore a few things). If I enable AMD Live, Cool&Quiet or other things from CPU configuration menu I'll be sure that the computer won't reach Windows desktop in 99% of cases (it randomly hangs somewhere in the boot process or even in the BIOS POST).

Another interesting thing is that during the POST process the computer always takes unusually long time detecting USB devices (LCD POSTer shows USB INIT), and I've also tried disconnecting all USB devices but didn't take less time to POST

BIOS revision is 2702, the latest. Today I found a different behaviour once: during boot screen I got a BSOD with error Stop 0x00000101 A clock interrupt was not received on a secondary processor within the allocated time interval, and this is usually related to overclocking, but I never overclocked my CPU.

Judging from the description of my problem, hoping someone had the same and fixed, and since I don't have a spare CPU or motherboard for replacement, I'd like to ask if you think this is a problem with faulty CPU or faulty motherboard, and if I can perform additional tests (I mean software tests because of my lack of spare components) to identify the component to replace.

There is little thing you can do now.

From my own experience, I guess it's the mobo that's the culprit, since I've seen much more faulty mobo than CPU in my life. It might just be a bad cap. Or some intricate problem somewhere in the board that's impossible to detect with any kind of software.

Of course, this is just my guess and it's unfounded. If you don't have spare components to test, it would be impossible to track down the culprit between the two.