Any update on this? I find myself getting the same behaviour and my parts are very close to yours, the only difference is that the system rock solid until ~2 weeks ago, possibly slightly more.
My parts are:
z68x-ud3h-b3
2500k
Corsair Vengeance 2x4GB RAM
Corsair 550 PSU (not quite sure on model, i think it's a VX)
Nvidia 570 GTX
Like you i've never gotten a crash in a game or a CPU/GPU intensive thing, in fact the 3-4 crashes that I've gotten happened when I was away from the computer.
The crashes started recently, and i've only gotten 3-4 in the last few weeks. The only recent changes I've done to my system was change my video card from an ATI 5850 to an Nvidia 570 GTX, however this was done over a month ago. I'm getting the crashes in Win7 and Linux.
Things i've done to try and fix it:
-update bios from F8 to F10
-disable USB3 in bios
-disable onboard audio in BIOS
-lowering OC'ing, originally at 4.2Ghz, now at 4.0 (scaling back slowly).
-clean out/dust case and rewire everything
-run memtest (did 6 passes, maybe i should do more tho).
-my CPU at idle is no more than 24C, and load no more thand 55C.
I was just curious how your system is now and if you did swap motherboards (and if so which one)? I was going to grab a newer CPU but i'm thinking of just grabbing a different board now and hoping to find out what you got. I'm also tempted to put the old ATI card back, but it was a $275 card which is more than the board or CPU that i currently have.