Honestly, you've been lucky so far, but the guy sold you a junked CPU. I'd probably leave the extreme overclocking tests in the past and just settle into a low-voltage overclock, perhaps 4GHz or 4.2GHz. Your heat spreader installation is insecure, and the TIM application method you used simply isn't going to be stable over the long term. Regardless of whether it was overnight or over a fortnight, the temperatures were bound to climb.
Sorry to be the bearer of bad news, but I'm not the only one delivering it. Better to just enjoy the system rather than pushing its (unstable) limits.