PCI-e power connector seems broken

I'm pulling an old mac pro 2,1 out from a closet and upgrading it to use as my main editing machine. Unfortunately, it seems one of my PCI-e power connectors may be broken. So far I'v tried two different graphics cards. One has a single 6-pin power connector, the other one has two.

The card with the single connector is rather unreliable and has a lot of dead pixels. (not just in this machine)

It seemed to work fine when plugged in to one of the connectors, but using the other connector a red LED turns on on the card and when I booted the computer to a windows vista system, it booted to just a black screen with a red warning text telling me that I needed to connect power to my graphics card.

The other graphics card I tried didn't work at all. I tried a few different monitors, but none of them got a signal. I installed realVNC and it did show up as a VNC server, and I was able to connect, but it just showed windows had booted to a square black screen with just a mouse on it.

All of that suggests to me that one of the power connectors on the board of my 2,1 is broken, but it seems like an unlikely thing to fail, since the machine works fine otherwise, so it's not the PSU that has failed. After a quick google, i found no on else experiencing the same issue.

Also, I don't fully trust either of the two cards I tested with.

The single conector card is clearley at the end of it's life and could just be broken, and the other card is not support on a mac pro, and has never displayed bootscreens or worked with OS X. (although it has worked in my mac pro 1,1 before when running windows 7)

So in short, my question is: Could it be the power connector on my board? Has anyone else experienced/heard of this issue before?

I don't want to go tear my machine appart just to find out that it was the cards after all.