Lower the clock or increase the voltage if your cooler can handle it would be the obvious answer, I doubt 1.3v is going to run 4.2ghz stable on a 6800k. Most people dont push that chip much past 4.0ghz for daily use, it starts producing a ton of heat with the voltage required to keep a stable clock past 4.0ghz on the majority of samples.
Are you just changing the multiplier and increasing core voltage? If so I would recommend you read a broadwell e overcloking guide, there are lots of settings you can tweak other than core voltage to help the stability of an overclock.