Maybe you have a bad core , you have to test each core , i was lucky enough that all my cores could do -20 and ive started from there, you can use PBO2 Tuner and tweak each core , maybe some can do -30 some can do only -10 (or 0 , i dont know).
EDIT: Some of my cores probably can do -30 (i tested some -28 and they were fine ) but -26 on most of the cores is enough for me , it was time wasting to tweak each of them by 1 for the gains i could get by doing that.
The core 0 with -20 is pretty slow in CPU-Z (i think CPU-Z is using only the first core to test single thread) but in CB the score is great , everything is stable and i see all the cores are boosting to 4850 , i am not sure why my first core doesnt get good score in CPU-Z , probably the thermals , it gets too hot with -20 @ 4850mhz and it throttles down.
Edit2: With this settings im getting 6800 points Multi and 655 single in CPU-Z , setting core 0 to -30 im getting 6850 / 662-664 without crashing during the CPU-z Bench but if im testing for stability it gives an error.
With -30 the C0 is boosting to 4850 and with -20 is boosting to 4800 when testing it in single core CPU-Z , that lets me thinking that is thermal throttling anyway at -30 its not stable in stress testing.
When ill have the mood ill try to set a higher LLC (its on Auto now) , maybe i can make it stable. (you can try changing the LLC too and see if that helps)
Edit3: After letting my computer idle some more with C-states enabled i see that the CPU downclocks even lower, check the pic. ( i think thats what u wanted? )
