Question New graphics card causing computer to boot into power save mode?

Aug 1, 2019
Hello all,
I just upgraded to a Sapphire Nitro+ RX580 8gb card, but the installation isn't going so smooth. I built this PC about 7 years ago, so some parts are starting to show their age, and I figured it'd be time for an update.

The problem is, when I plug the power cables (a 6 pin port and an 8 pin port that I'm using a 6+2 cable for) into the GPU, it causes my monitor to boot into power save mode when I turn the computer on. It doesn't seem to have a problem with the card being plugged into the motherboard, so long as the power cables aren't plugged in.

I'm wondering if it's a compatibility issue? My specs are here:
Processor: Intel Core i5-3570K CPU @ 3.40 GHz 3.80 GHz
8 GB of RAM (7.90 GB usable)
Motherboard: Gigabyte GA-Z77-DS3H
Currently running Windows 10

Would any of these be incompatible with my new graphics card? I don't have the PSU model right now, but I can find it if anyone thinks that could be the issue. I know it's at least 600W though.

Anything you can offer would be great. Thank you!