As I suspected this looks like a shortage of power issue.. I checked out your power requirements and the min. power requirement seems to be 850W..
XOV PSU Calculator component list
Motherboard: Desktop
Socket: Socket LGA 1151
CPU: 1 x Intel Core i5-6600K 3500 MHz Skylake-S
CPU Speed: 4500MHz
CPU Vcore: 1.42V
CPU Utilization: 90%
Memory: 4 x 8GB DDR4 Module
Video Card Set 1: 2 x NVIDIA GeForce GTX 980 Ti
Core Clock: 1000MHz
Memory Clock: 1750MHz
SLI / CF: Yes
Storage: 2 x SATA 7.2K RPM
Storage: 2 x M.2 SSD
Other Device: 2 x USB 3.0 Device
Keyboard: 1 x Standard Keyboard
Mouse: 1 x Standard Mouse
Fan: 4 x 120mm
Liquid Cooling Kit: 1 x Corsair Hydro H110i GTX
Computer Utilization: 16 hours per day
Load Wattage: 797W
Recommended Wattage: 847W
Amperage: +3.3V: 11.6A, +5V: 12.2A, +12V: 62.3A
Recommended UPS Rating: 1440VA
Generated by eXtreme PSU Calculator 2016-05-30 04:24:27
Another power calculation by PCPARTPICKER.com.. Even without overclocking the PCPP list is showing a wattage requirement of 759W..
PCPartPicker part list:
http://pcpartpicker.com/p/LwWj4C
Price breakdown by merchant: http://pcpartpicker.com/p/LwWj4C/by_merchant/
CPU: Intel Core i5-6600K 3.5GHz Quad-Core Processor ($233.99 @ SuperBiiz)
CPU Cooler: Corsair H100i v2 70.7 CFM Liquid CPU Cooler ($99.99 @ NCIX US)
Motherboard: Gigabyte GA-Z170X-Gaming G1 EATX LGA1151 Motherboard ($464.99 @ SuperBiiz)
Memory: Crucial Ballistix Sport 32GB (4 x 8GB) DDR4-2400 Memory ($119.99 @ SuperBiiz)
Storage: Samsung 850 Pro Series 512GB 2.5" Solid State Drive ($219.21 @ Amazon)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($53.50 @ Amazon)
Video Card: EVGA GeForce GTX 980 Ti 6GB Superclocked+ ACX 2.0+ Video Card (2-Way SLI) ($609.99 @ NCIX US)
Video Card: EVGA GeForce GTX 980 Ti 6GB Superclocked+ ACX 2.0+ Video Card (2-Way SLI) ($609.99 @ NCIX US)
Total: $2411.65
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-05-30 04:30 EDT-0400
I think All your issues will be solved if you upgrade the PSU to atleast an 850W unit, you can practically verify this by using an 850W PSU if u can get a spare one..
PS. I am pleasantly surprised at the quality and endurance of the EVGA G2 series that it has managed to run the PC this far, other PSUs would probably have quit long-ago..
🙂)