@1day, got the same problem here. it does 5 quite easy, after this everything changes.
i can bench 5.1 at 1.425 in bios... for 5.2 however i need 1.5v in bios and it locks during os loading.

But i figured out what my problem was/is. When i set 1.450 vcore in bios it drops down to 1.385 during load... seems like load line calibration is not working or something.
The best thing, dust off your fluke and measure the actual voltages.