Right, play around, you can up the voltage to 1.35v-1.4v max so keep trying, not all CPUs can handle high overclocks, maybe youve hit the limit.
But try to play around this the LLC %... Start lower and raise it and see if you become more stable (Load-Line Calibration will give you extra power for overclocking when your CPU is at load and needs it)
Go upto 1.4v, find your highest overclock and leave it at that.
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As above, you can usually squeeze more out if you play around with the vcore and LLC. However, when you reach this "wall" why not keep it at that point, as the cpu is "happy" - the voltage is low, the speed is good. Most of the time its not worth the extra voltage and heat for another 1-200mhz IMO.