My 4770K is only happy up to 4.2GHz too, also on a Gigabyte GAZ87X-D3H but in my case it's only on 1.18v Vcore, so runs cool with my Noctua NH-U14S. When I push past 4.2GHz and I've tried 4.3GHz, 4.4GHz and 4.5GHz and Vcore up to 1.28V it boots ok but will fail P95 or eventually just blue screen (0x124 error) while doing something else, not even necessarily pushing it hard. I did just try increasing the speeds and leaving the voltages on auto, which went well over 1.3V but the results were the same. I do a lot of 3D rendering, which involves the CPU being used hard for hours on end, so I decided to stick with my lowly but cool and very stable overclock at 4.2GHz @1.18v. Max core temps are between 58C and 62C after a couple of hours or more of rendering and gaming rarely breaks 50C - I can live with that.