what motherboard is it? If there is another Pci-E slot you can try the GPU in that slot. I would also pull the cpu cooler, re seat the cpu then clean and reapply new thermal paste.
Also if you have platter drives i would run some test on them to make sure they are alright. Each manufacture has their own testing tool for their drives
Thanks for the suggestions. I don't have a CPU cooler (it's my brother's PC).
My motherboard is the following:
Gigabyte Z97P-D3 Intel LGA1150 Z97 ATX Motherboard (4x DDR3, 4x USB3.0, 10x USB2.0, GBE, LAN, HDMI)
So I can reconnect the GPU in the PCI 4.0 slot but is there a point to that because I need to connect to the x16 anyway.
I will try to reconnect the CPU tomorrow, maybe it's what's causing the issue. I'll update the thread tomorrow.