Very interesting.
Unfortunate thing is, I did switch to 'Performance Mode' on that inauspicious CPU Margin Enhancement setting at least once in the past - and that did indeed, similar to Big Lar, screw up my perfectly awesome OC. At that time I didn't need to re-flash BIOS, I just set it back to 'Optimized' and my OC was back to stable again.

But I get the feeling, just like truehighroller & Big Lar, the damage was permanent. Not that my Shut down issue started immediately after that incident - but whatever damage it did to the chip, it probably stuck.

I'm still uneasy about the fact that my Shut down issue started all of a sudden one fine day - when I was not messing with any settings and everything had been running stable for a while.

I still have to test my Stock (Defaults) vs. OC theory later today on 1403.