sup Sergio, I had that problem too as you know a while ago. It was the first symptom of electromigration with my crucial sticks.

http://www.xtremesystems.org/forums/...d.php?t=140870

The weird thing was as in your case that the system was rock stable at memtest, orthos large, OCCT, but once you power off you could not start it again.
Try to disable Dram Static Read Control, that should loose the performance level a bit, and hopefully not so stressfull for your RAM.