Can occur due lack of power on the power riser, try to change with different power riser like 6 pin/4 pin /sata or can be happened because of pciex1 or a cable on riser is broken.
So i very confuse went i use this gpu with x16 on board (not use riser card) the vga is narmal.Also i try with another computer with x16 on board the vga is narmal.
can you try to PCIex16 Riser?. I doubt a GPU only want PCIex16. but no problem if you want to try that. like this:
https://www.amazon.com/EZDIY-FAB-Express-PCIe3-0-Flexible-Extension/dp/B01CEKN5VO/ref=sr_1_1?ie=UTF8&qid=1520574847&sr=8-1&keywords=pcie+x16+riser