PC locks up during stress but not overheating

My PC has been locking itself up without bsod and putting painfully distorted sound out through the speakers, or just silence. This has been going on for a while, and it seems like the problem is beyond my abilities to figure out. It tends to happen more during gaming, but it has happened while watching videos or idle as well. I don't play particularly demanding games, just stuff like DotA 2 and Civ 5. I'm not overclocking anything, by the way.

Here are the things I've already attempted, with no success:

-installed open hardware monitor and speedfan to watch temps during gaming and stresstests, gpu peaked at 62c and cpu at 55c
-swapped to a clean hdd, and a fresh win7 install
-updated bios
-Updated every driver I know of
-ran driver verifier for a few days
-Ran memtest+ for 24 hours with all sticks in, then each stick individually for about an hour each. Also tried every ram slot. No errors, but when a stick was only in a channel B slot it ran at 200mhz regardless of bios settings.
-Manually set ram speed and voltage to manufacturer specs in bios.
-Stressed gpu with furmark for over an hour, temp peaked at about 62c, and was completely stable
-Ran Prime95 small and large FFTs, always freezes within a few minutes.

2 or 3 times, the pc would not post after a hard freeze. After I pressed the power button the fans and lights would turn on, but it wouldn't POST; I just had a black screen. I let it sit for a few minutes with the fans and such running, and it eventually started itself normally. Unfortunately, upon closer inspection I realized that my mobo speaker was not plugged in, so I didn't get any beeps. Fixed the speaker, but it hasn't had boot problems since.

I just need to know for sure which piece of hardware is configured wrong / failing.

mobo: Asus m4a77td pro
cpu: AMD Phenom II X4 975 3.6ghz
gpu: MSI R6850 Cyclone PE Radeon HD 6850 1GB 256-bit
ram: 4x G-skill Model F3-10600CL9D-4GBNT
psu: SILVERSTONE Strider Essential series ST60F-ES 600W ATX12V / EPS12V

Can you post it following the Blue Screen of Death (BSOD) Posting Instructions? It may help us to have a deeper look.