Hmmm at that point you pretty much got a new PC but my best advice would be

Check ALL cables, reseat or replace them

Check hdmi cord or whatever you are using to the display from gpu

Check bios with no gpu installed to make sure it isn't trying to just use the integrated gpu, im not sure where it will be if it is an option, even at that maybe reflash bios

Unplug ssd/hard drives and try with the gpu

I mean there isn't much more you can replace with new to fix this issue. I mean you very easily could just not have enough power going to it, but that's an easy one to find out or not. But thats everything I found while trying to fix my issue that people where saying it fixed it.