Very interesting, can you try connect your gpu directly to x16 pci-ex slot ? for this you need unscrew your mb and test on desk
I unscrewed my MB and connected my gpu direct to x16 slot. Now,
if i plug my monitor to builtin vga it says
Attention: unsupported video configuration detected
And ask to plugin cable in gpu and turn on computer.
And if plugin the vga cable thru adapter to gpu's dvi, system get started but nothing displays on screen.