My 970 never wants to go to idle even at 120Hz. Or well, never is not entirely true. When I got the card it idled like it should, then somewhere down the line it decided not to. Then I tried to fix it and after many driver uninstalls and whatnot I got it to work for a while, but didn't take long for it to never idle again.
Some say the problem was because of dual monitors and I could get it to clock down when I disabled one monitor back then. Now it does nothing and it stays at 40% power no matter what.
I have found one solution that works to force it clock down, in Nvidia Inspector you have this which can force it into idle clocks.
The problem is that it can mess with overclocks if you enable it to run at startup, it activates before afterburner and afterburner can't apply the clocks after that or something.
This shit is a bit annoying since I know the card can run at ~35C while idle when it works properly and it sucks that I have to use some software to fix an issue that is there for no real reason at all. There's absolutely no problems running 2 monitors (one of them at 144Hz) at idle clocks.
So currently I am sticking with this power saver thingy with GPU usage threshold set at 30%. Keeps the clock down on desktop and got no problems with it in games. Only thing is that I have to remember to run it manually after a reboot.