Intel Motherboards : Poor OC / PSU Problem

As for the overclocking, your board is a solid one so in all likelihood you just have a poor overclocking CPU. My 6600k is also only properly stable at 4.4ghz, I can get it all the way up to 4.6ghz (like the validation in my sig) but it isn't stable and needs a lot of vcore. Overclocking is a very luck oriented thing, that is why some stores actually sell CPUs that they have tested stable at certain overclocks for a lot more than the retail value of the CPU. 

Google "Skylake overclocking guide" and see if you can find any info you have missed. On my system I find the easiest way to overclock is to set one of the EZ-OC modes (usually the lowest one) then adjust the multi higher until it becomes unstable. Once that happens I bump up the vcore 0.05v and try again until stable or I get to 1.4v at which point I stop and back off a little on both the multi and vcore. I would start with a fixed vcore of 1.2v and say a 42 multi and work my way up from there. Setting one of the EZ-OC modes sets a lot of things automatically for you like short and long power limit, LLC etc so you don't need to think about them. 

Good luck and let us know how it goes.