Man of Honour
My only advice is to swap out that PSU.. you've swapped out everything else so as you say I can't be anything else..
also, I know it's a different graphics card.. but can you try the graphics card in another machine.. I'm wondering if something is borking it... i.e. damaged the first one.. then the second.. although highly unlikely, it's worth investigating and eliminating.
Shame you didn't live nearer.. I'd ask you to bring it in to the workshop.