MB, PSU, short ?

[Bartmole]

I built a pc about 2 months ago with an Asrock Extreme6 X99 and an I7 5820 and updated the Bios to the latest version (1.90), also a mild overclock of 4.0 at 1.115 Volts. During that time, I've experienced a few freezes (1-2 a week with 10-12 hours a day use), almost always at the Windows loading screen when leaving the Bios. I had an NH-U12S with dual fans and temperatures rose from 34C to 50C in the Bios, probably after the update, so installed an NH-U14S with dual fans and see a consistent 56C in the Bios, Core Temp and Aida shows around 23C at idle and 60-68C at 100% after 3 hours and the boards other Bios with an older update (1.50) shows 37C, still not where I think it should be but close enough. Still getting the same freeze at the same spot but not as much but my main concern is this. Used the pc one night and shut down and in the morning it wouldn't boot, not even the fans. Tried a few hours later and it booted to Windows and instantly rebooted it self but died completely before loading Windows, 3 hours later, it again booted to Windows and ran normally for about 15 minutes before dying completely again. Nothing using the MB start button but fiddling with the on/off  switch in the PSU did get it going a few times but only for a few minutes. I have a second computer with the same PSU (Corsair 760 AX Platinum) so I switched PSU's fully expecting the second computer to act the same but now both have been running fine for days, even under stress testing, and I'm not sure what to expect. Sorry for the long post, not sure where to look anymore. Oh yeah, Memtest was fine, too. Thankyou

[itachipirate]

Make sure all of your motherboard screws are screwed in. Have you tried clearing CMOS?

[Bartmole]

Yes, cleared CMOS and checked all cables and connections, only thing that seemed to even help a bit was fiddling with the on/off switch on the PSU. Feels like I'm just biding my time until it fails again.