Since writing I've found that the issue is that CPU, which I've RMA'ed with Intel. Swapped it out with a 6800K from a friend's PC and everything worked sweet. Thanks for your help!
Hi,
Here's the specs:
ASUS ROG Rampage V Edition 10
Intel 5960X w/ Corsair H105 AIO & Noctua Fans x2
Corsair 16G x8
GTX 1080TI MSI SeaHawk x2
Corsair RM1000X PSU
OS is Windows 7 Pro
The system was running rock solid stable up to around a month ago. The CPU was modestly overclocked to 4.2GHz...