Have you looked in the BIOS? I don't own that particular board, but it shouldn't even support x2. If the BIOS doesn't help I'd yank that sound card (I saw the other thread), and test the GPU in each one.
I had a similar problem, I just did some cleaning for those pci-e slot contacts on the video card (small golden ones), the chemical liquid I used was a nail polish remover, yes, it's good for cleaning electronic chips too, I just don't remember the common name for that liquid... so and then reinstalled my video card back to the slot, double checking that video card is firmly seated and its power cables.
After that it's at x16 again!