Had another little fiddle today, and almost got CPU stability at 4GHz (211x19) on 1.25vcore and 1.34vtt, even booted at 222x19 (4.2GHz) but with stock cooling, stress testing was a no-go. Backed off to 211x19 for 4GHz and after a couple more tweaks, I'm hoping to return to work with a 'closer to stable' machine tomorrow morning. The one thing I noticed when running ram bandwidth everest benches is that it scored very low - i.e ~6000MB/s on read/write/copy. A little unfamiliar with what could be happening, but I'm guessing uncore multi might be out - that possibly correct? HT is on btw.
I'll post some screenies up if all goes well tomorrow. It seems like I've got a good board & chip which is gagging to go under water!