I have a pc that I've never had a problem with before. The last time I made any modifications was updating the graphics card and adding a usb card about 2 years ago. The specs are:

Core i5-4670K 16GB ram (G.Skill ripjaws lp) MSI GTX970 Corsair RM750 MSI Z87 GD65 gaming Windows 10

A few days ago, I turned it on and just got a blank screen. No post. However, there is sometimes the notification sound that Windows makes when it's detecting usb devices, so it sounds like it's booting up but I just can't see anything.

After a couple more reboots, I tried clearing the CMOS. No help. Then I remembered the board has two BIOS, so I switched to the second on. Still no help. I then tried taking out the graphics card and plugging my monitors into the on-board graphics, and it booted up fine. I thought maybe the card just wasn't outputting the video signal, so I put the card back in, and got no signal again, even with the monitors plugged into the on-board graphics still.

So, now I was pretty sure it was the graphics card that was the problem, and was getting ready to check if it's still under warranty, but I thought I'd better make sure, and I put it in my girlfriend's computer, but it turns out it works fine. I put hers (a GTX 1050) in mine, and have the same problem as before---no signal.

So, now I have it narrowed down to motherboard or PSU. The thing is that her card doesn't use the external power connectors, so if it's a PSU issue, it would mean it would have to be a problem with the power that's going through the PCI-e slot. I'm not sure how likely this is.

It's also worth mentioning that I tried the card in another slot in my PC, and it still doesn't work, but my USB card is in a 1x PCI-e slot and works fine. So if it's a problem affecting multiple PCI-e slots, it appears to be only affecting the 16x ones.

As a last resort to help differentiate between a motherboard/PSU problem, I can swap mine with my girlfriend's and see which one boots when the only thing that's swapped is the PSU, but I'm sure everyone agrees with me that doing that is a bit of a pain in the arse; I'd prefer to avoid it if I can work it out without doing so. (see edit)

Does anyone have a good idea of whether I'm right in guessing that it's probably the motherboard? Is there any way I can do further diagnoses with software? Something to look at voltages perhaps? I don't have a multimeter. Is it unusual for a fault to cause two 16x slots to stop working while the 1x slot I'm using works fine? Or, on the other hand, is it possible for a PSU fault to cause this problem even with a card that doesn't use the external power connectors? Is anyone familiar with this particular set of symptoms?

Any help would be greatly appreciated. Cheers.

EDIT: Tried my girlfriend's PSU in my PC. Same problem as before; works fine with integrated graphics, but no signal to any displays (including integrated) when graphics card is installed. This makes me almost certain I'll need to replace the motherboard, but I'll leave this open in case anyone has any ideas.