I used to have my ram oc'ed to 2600Mhz, But a few months ago it just stopped working at that speed and I had to set it to 2133. My Ram is Patriot Black Mamba 2133Mhz DDR3 8GB Kit. The only setting I set to get my ram to work at 2600Mhz is the speed, I just set it to 2600Mhz in the UEFI and it just worked. No extra voltages etc. I have tried loser timings and higher dram voltage with no luck. I'm getting a "55" Error code when setting the ram to 2600Mhz witch according to my mobo manual is memory fail.
Could be as much OCing it could handle, Ive heard many stories of CPUs etc no longer holding their long time OC, hardware starts to fail faster when running faster than it is designed
Also It could be the memory degraded to the point where it cant be OCed past 2133 but you could try 2400 and see if that keeps. Have you tried timings that go with 2600mhz RAM? That would be my next step before trying 2400.
It could also be the IMC on the CPU showing its age. The more your overclock, the faster it degrades, this could just be the first thing to give out. You might get it to work with more voltage however, but that'll further degrade components.
Well, I guess it's not sold as a 2133Mhz kit for nothing, and 2600 isn't a small overclock. My guess would also be, that your ram (or cpu or imc) shows signs of aging. Sorry, but I'm afraid you're out of luck on that 2600 oc...
tweak ram current if you have it (from stock to 110 or 120%), and adjust cpu vDIO - digital IO voltage, also VCCSA.. vDIO ~ 0.090 - 0.120v+ offset VCCSA ~0.030 - 0.050v+ offset