Overclocking on Intel Haswell processors is quite a bit different from what we are used to in the last few generations, where the CPU clock speeds did not influence the clock speed the memory controller could handle. Entry to Haswell memory overclocking 101 shows that while the CPU may handle a nice overclock of up 4.7GHz to 5.0GHz on really impressive examples, the memory may not scale up past 1866MHz or 2133MHz at those CPU clock speeds, even though the memory modules are rated much higher. What I found on both of my CPUs was that anything greater than around 4250MHz on the CPU would not run the memory at 2600MHz or higher. The G.Skill Trident comparison modules would run just over 2600MHz at 4.4GHz, but the Black Mamba Viper 3 modules would cause a lockup at that speed. After plenty of trial and error, I was able to push the modules up to 2800MHz by relaxing the timings to 13-15-14-35 and letting the motherboard assign the rest of the parameters.