Same board with es 2660 cpu's and G-Slill ram here.

Cannot give you a definitive answer but I offer some observations.

After a week running WCG, on re-boot I get OC failed. In view of the fact that the only OC is force ram to 1600 I wonder if the standard (auto) settings for ram secondary timing or possibly imc voltages are not quite up to the mark. For me even primary timing is something of an issue which to me seems to confirm this.

It is the area that I will investigate first once I get a chance to run windows instead of Linux.

Maybe referring to some of the overclocking boards threads for voltage settings and secondary timings will lead you to a solution.