BadTimetoFail

So I've tried a new PSU and there's no display with a dedicated GPU. I've updated the bios and forced dedicated GPU in the bios as well. The RX 580 fans don't spin on boot at all so does this suggest the PCI-E slot is faulty? The 7870xt fans do spin on boot however it has two 6 pin connectors were as the RX580 only has one 8 pin. Does this suggest the PCI-E slot isn't providing enough power?

I have a RM550X arriving Sunday to check the PSU, but is there a chance it could be an issue with a PCI-E lane? I have tried both lanes on the motherboard and both have the same issue.

My old graphics card (HD 7870XT) had been going strong for 7 years but no longer displayed so I bought a RX 580. It worked for about 20 minutes in game (albeit very choppy) and then the screen artifacted and went blank. It won't display on boot like the old GPU. When I remove a dedicated card and use the integrated graphics it boots. If I connect the RX 580 and force IGPU on the bios, it boots. If I connect the 7870 XT and use the IGPU it gets stuck on a bootloop. Does this suggest that the PSU can't provide enough power? The 7870XT fans are running on boot even when stuck on bootloop were as the RX580 fans are idle when booted with the IGPU. The PSU is an XFX 850w bronze rated PSU but it is 7 years old.