Can't get 6 1070s stable

Just testing out my rig setup in preparation for my motherboard to come in stock and be shippped to me, so currently have 6x GTX 1070s on the mining rig powered by an EVGA SuperNOVA P2 1200W PSU. I turn this on manually and use the included cap (PSU tester) to get the PSU to start as if connected to the motherboard. Is this safe?

My test motherboard is my gaming PC's Gigabyte Z170X-Gaming 7 motherboard powered by an EVGA SuperNOVA G2 850W PSU. I turn this on via the PC case's power button. This PSU powers everything except the GPUs and risers.

I can only seem to get 5 cards to work with this setup. I've tried GPU #5 on the last 2 PCI-E slots and it works and 5 GPUs are detected each time. I've tried GPU #6 on the last 2 PCI-E slots and it works and 5 GPUs are detected each time. This confirms that the GPUs, risers, cables, 1x chip, and PCI-E slots are all working. However, when I populate all PCI-E slots, the PC simply starts up, I hear a "tic," and the motherboard reboots.... a few seconds later, same cycle again. Remove one GPU from the setup and the PC starts up no problem.

Any suggestions? I'm not sure what I'm missing and I believe I've exhausted all possibilities now. I do have an M.2 NVMe board installed... I had two boards but the manual stated that one PCI-E slot would've been unusable so I took out one board and relocated the other board to the "safe" PCI-E slot but still no joy.