I had the same problem with gtx 1060 (hynix). Install the latest updates, set the speed to Gen1 and try again. It should do the job. and yes, use powered risers.
I tried this but the issue is the same.
Yes I use powerd risers.
If I connect two 1060 directly to die PCI-E connector (without riser) it works fine.
Has anyone an idea how I can check if the risercards work correct?