Hi , i have the zotac version of the 1080 , this might be a dumb question but do your fans spin for a few seconds at boot up and shut down , if yes then you can rule out broken connectors.
When i thought i had a problem i took the side off my case and ran the built in bench test on Horizon Zero Dawn , i put the task manager "on top " so i did not have to tab out of the game and the fans kicked in at 70 and never went above 76 , are yours even turning on.
Get rid of the nvidea experience bit of software if you are using it , it is a pain in the **** , when you update the driver choose manual install and untick the experience box or you will be given it again.
Are you using the overclocking program that came on the driver disc , you dont need to , the makers of my card said you dont need it as the 1080 has the ability to do the equivalent of overclocking itself when put under load.
Have you ever cleaned the inside of your pc to get rid of dust on the fans ( and the bearings ) , with the pc turned off use a can of air known as an air duster to clean everything.
WARNING..... no matter how awkward it is to get at some parts DO NOT turn the can side ways or upside down or the propellant with squirt out with the air , also use it in short bursts because some cans can suddenly become very cold and burn your fingers.