GPU – Why Is It Showing x1 Bus Interface Instead of x8?

TL;DR: GTX 980 is running at x1 instead of x8 both in SLI and as a solo card. Another 980 is running perfectly in the same situations.

I recently purchased a second 980 (EVGA) to run SLI with my current 980 (MSI). Installation went smoothly, with Windows and NVIDIA recognizing the card, installing the correct drivers, NVIDIA enabling SLI, etc.

However, upon loading into a game, I was getting terrible performance — like, 1-2 fps (not an exaggeration). I did some digging, pulled up GPU-Z, and discovered that my EVGA card is sitting at PCIe x 16.0 3.0 @ x1 3.0, rather than at x8 like I expected (see http://gpuz.techpowerup.com/19/03/06/uye.png).

I tried swapping the cards, but again, EVGA shows x1, MSI shows x8. I also tried running only the EVGA card (removing my MSI card from the system entirely), but it still shows x1.

Speccy shows the same results as GPU-Z.

I also tried updgrading my PSU to a 750W, in case power was in issue, but I'm seeing the same behavior. GPU-Z reports when the card is under load that it isn't getting enough power. Also, the fans never turn on.

Specs:

• CPU: i5-4690k
• Mobo: MSI Z87-G45
• RAM: 16 GB DDR3 1600
• GPUs: 2 × 980
• OS: Windows
• PSU: EVGA 750W

I've tried the following:

• Reseating the card (like 20 times)
• Cleaning the card and mobo
• Swearing furiously
• Clearing my mobo's CMOS
• Flashing GPU BIOS
• Uninstalling and reinstalling drivers

Some interesting behaviors I've noticed:

• The EVGA card swaps between x1 1.1 and x1 3.0 in GPU-Z, seemingly at random. Running the fullscreen/render test sometimes shows 1.1, other times 3.0.
• With the EVGA card in the top mobo slot (x16, 16 wired), NVIDIA Control Panel doesn't recognize the SLI bridge.
• I put the card in a friend's system and we were able to get x8 bandwidth on it immediately (in a multi-GPU setting), indicating a problem with my system.

What could be causing this? Any ideas on a fix?