I've been up super late for almost a week trying to figure out #### is going on with my brand new build. I reckon I tried EVERY possibility possible (except for plugging the GPU directly into motherboard) and had the same issue every #### time. I changed the PCIE Mode on slot 1 to GEN 2.0 and it worked for a day... which I was pretty happy about but not 100% satisfied.
Thanks to everyone who has raised this issue and recommended the 3M cable as that is the one I have ordered. I have complete confidence this cable will work without any issues.