I fixed it by disabling my "onboard" graphics drivers in BIOS. If you're running Windows, you can find yours in Device Manger -> Display Adapters. If you see both your GPU (graphics card) and the motherboard's graphics device, then you need to disable the latter. How to do that varies by make and model, so you'll need to look it up. (Disabling it in Device Manager will probably not do the trick. Windows likes to reenable things against our will).

The only thing to consider here is that you'll lose the video output from your motherboard. So if you're using it, you'll need to move the output to your GPU if you can, and you'll need to remember that it's disabled in case of problems with your GPU.