I read somewhere that if you lower CPU PLL this can actually help with stability, I have mine set to 1.300v instead of 1.800v, I am now stable at 4.2GHz, I have spent the last 2 days getting it Prime stable though, it has been a real PITA!!!

I need really high Vcore for it to be stable though, 1.3500v in the BIOS, CPUZ says 1.312v under load though, my QPI/Vtt is at 1.335v, and IOH 1.160v, I ran P95 Blend test for 6 hours earlier, and it was fine, however I am going to leave it running overnight just to be 100% sure.