Random Black screen and crash

Hello everyone,

I have been experiencing random crashes on my newly built pc, and so far, every atempt I've made at trying to find a solution have failed.
At first, it seems to be quite random, but some program seem to make it crash every time I use them.

The symptoms are as follows:
-black screen (though sometimes it shows colored stripes instead of total black)
-hdd still active for 5-10 seconds, then no activity light whatsoever
-sound loops
-even num lock is unresponsive
-a signal is still sent through the graphics card, as the TV does not show the usual "No input signal" when there's..well, no input.
-no reboot, I have to manually reset the PC
-no dump files

My setup is:
-MSI P67A-C45 B3
-i5 2500k (not overclocked)
-2x2Gb G.Skill Ripjaws X 12800 cl9 (not overclocked, with and without XMP profile activated, but I had to set the right timings in bios as it showed 9,10,10,29 instead of 9,9,9,24)
-Sapphire HD 6850 (not overclocked)
-Antec HCG 520W
-WD Caviar Blue 1TB
-image is sent to a LG 32LH3000 LCD TV via HDMI

I've tried everything, from reinstalling graphics drivers (old, new, preview 11.4...), to reinstalling windows 7 (I'm on the 64 bits version).
It does not seem to be a hardware failure since every test or burn I make is succesful, and does not cause any problem.
Graphics card does not go over 70°C with Furmark, and handles every game perfectly.
CPU does not go over 75°C under Primer 95.
MEMTEST did not show any error during the 11 passes it did, and I also tried switching the slots in which they were installed.
I did a speedfan log along with a Furmark log until it crashed, and they show correct readings.
PSU delivers good voltage on the 12V (12.225V in BIOS).
Heck, I even tried on a different wall plug, in case the one I was using was faulty!
Nothing seems to correct my problem.
One thing I noticed: I tried launching Bioshock 2 (Steam version). It runs perfectly when I play immediatly, but whenever I try updating it (via Game for Windows Live), it crashes anytime between login, and installing the update...

But this crash is not game related: it did have the same crash a few times while on the desktop, doing nothing special. The first time
this black screen appeared was when I switched from Basic Windows Theme to Aero after first OS installation.
It also happens during loading screens or menu in some games (Shogun 2, BC2...) but never happens while gaming
Anyone knows what could be the cause of this? It is very annoying, frustrating AND worrysome, because I don't know if it is a hardware problem, or a software one.

A few questions:
-could this be caused by the TV? Or the cheap HDMI cables I am using? I've been using this on my laptop previously, and never had any problems.
-Bad OS installation? Even though I installed it twice already...Maybe the DVD is damaged (i burnt it a while ago), but the OS still installed even though it had errors?
-64bits version?
-Graphics card failure?
-software/driver conflict?
-anything else?

Thanks for having read all this, and thanks in advance for helping me, I really don't know what to do...