A friend and I both recently purchased the exact same set-up. Gene XI M/board, CPU 9900ks and ram (as in my sig) both with custom loops.
Mine runs 5.2GHz no problem and might be able to go higher but his struggles to get past 5.0GHz. He has been having similar issues as you with the blue screens etc if he tries to pass 5.0, even with pumping the volts and trying higher llc. We settled on 6 for llc and raising the idle voltage, as higher llc just seemed to make it worse. I even tried saving a .cmo file to usb with "safe" settings on my pc and sent it to him, but that didn't work either.
I get a higher reading in HWinfo than cpu-z also but not much.
is alluding to makes good sense and I might look into it for my mate
Maybe with the advent of the 9900ks chips it's just harder to get a decent 9900k?