Dell Precision T3610 Install GTX 1080 (8pin PSU cable )

hi i have  Precision T3610 , and buy

  VGA  GTX 1080 ,

I plug new cable PSU power PCI  

cable1

8pin to 6+2 pin

and  pc not start,   POWER AND number led  123,  blinking 

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Buy a new cable 

8PIN 6+2  

PC STart but  in Scren apers 

"Please power down and connect the pcie power cables"

I nedd change other Hardware ? 

i not found in internet cable  8pin to 8 pin . 

please :) 

Emerson Manfrin