Hi guys, really hoping someone has the time to help out, and I hope this post can help others having any similar issues. Much appreciated .

Long time xbox-er, finally took the plunge into building a PC after years of back and forth and GPU shortages, but for some reason my system currently won’t boot if the 4080 is plugged in. Without the 4080 plugged in the system boots fine using the on board CPU graphics, and I’ve gone in and installed Windows, all drivers, intel chipset etc.
Spec -
H9 Flow Case
I9 13900k
MSI Z790 Gaming plus WIFI
Gigabyte AERO OC RTX 4080 GPU
Deepcool LT720
Crcuial P5 2tb NVME SSD
Corsair Vengeance 2x16gb RAM (Only one of these sticks work so 16GB currently, tried moving the stick into known working slots, still not lighting up etc, either a dodgy stick or another issue to resolve)
1000w X series Corsair PSU
Samsung Odyssey G7 Monitor - 240hz/240FPS
Tested both with HDMI and official DP cable that came with monitor.
Initially, even without the GPU installed, the system wouldn’t post/boot. Flashed the bios with latest version direct from MSI and this then allowed me to get as far as the bios without the 4080 in the case. Went on and installed Windows and everything else, but if putting the 4080 back in, I get no output to my monitor whatsoever and nothing will boot.
The 4080 has power going to it via the included 12VHPWR adapter cable (3x8 pin PCIE), it lights up, fans spin initially then stop, but it stays illuminated and I just get nothing to my monitor/tv display.
FURTHER TESTING
Just did more tests again, so without the GPU in, everything boots fine, can get to bios, windows etc.
As soon as I plug the 4080 in and turn on the system, there’s no output to my monitor or even an old TV I tested with.
I get the white light showing on EZDebug LED’s on mobo, the GPU fans spin for maybe 5/6 seconds then stop, the lights on the 4080 illuminate and never go off at all, but there’s no output signal coming from my machine whatsoever as if there’s not even a HDMI/DP cable plugged in.
Some research online has seen people mention it may be the 12VHPWR adapter cable causing the issue, as a change of cable seems to have sorted things for some others. Others have mentioned the NVIDIA firmware update tool could be a potential fix too, but unable to run this as unable to access the system at all whilst 4080 is plugged in.
This is my first build so if anyone could help in any way, that’d be absolutely amazing as I’m a bit stuck as to where to go next. Cheers legends.