Specifications:
Asus Maximus VIII Gene
Intel i5-6600k
16 GB - Corsair LPX DDR4 3200 (Running stock 2666Mhz)
Samsung 950 pro ssd
Seasonic X750 PSU
Corsair H90 Cooler
EVGA GTX 1060 SC

All fans were profiled by AI Suite and I'm using the standard setting
All drivers and firmware are the latest

Problem

Whenever I'm gaming the computer just completely freezes. It happens pretty quickly. If I'm not gaming and just generally using the computer it also completely freezes but this time it takes much longer. When this occurs the fans appear to ramp down slightly, the Power/Reset buttons on the case become ineffective. The only way to resolve is to switch off the power. If I restart the computer right away it can take a few attempts to actually boot (stalling at various phases according to the QCODE readout). If I launch straight into the BIOS Screen It still freezes in bios. If I leave it for a while it will start to work again.
Oddly - If I I'm running Prime95 or something it'll happily continue with no problems.

If I ramp up all my fans / GPU fan it seems not to crash or as quickly so I'm assuming this is heat related however when I run HWMonitor all the temps are normal and nothing is above 43c. I had these issues a year ago but presumed it with my GPUs fault as ramping up the fans on my GPU to 70-80% resolved the issue. I've since gotten a new GPU though and the problem is back. Now that I'm not running my GPUs fan so high it seems not to work.

So: I'm presuming either the CPU or Motherboard is at fault here (The PSU was RMA'd before). The Intel diagnostics show the CPU passes all it's tests so is there a way I can prove the Motherboard is faulty?