I don't remember what I've done when I upgraded my machine with the same GPU, but the 8 pins connector is convertible to 6 pins for information (look closer). But I didn't remember there was more supplying connectors on the motherboard (G20CB).
So, try to supply the GPU with all the connector from your motherboard, the GTX 970 need 145W whereas the 1080 requires 180W to run (concerning the Founder's Edition).
What is your card precisely? More specs about your tower?