I had a "similar" problem with 10900k + nh-d15, and what I figured out was that I didn't screw the heat sync tight enough. Maybe you should tighten it up as well. The reason I didn't do it right is cause when I was watching a tutorial vid on youtube on how to install it, he said that you gotta screw it in very gently and bla bla bla and so I did it too gently appereantly lol.
The first time I did it, when it was loose, there was a lot of give on the spring, in fact the spring was barely contracted. The way I redid it was making so that the spring is rather squished, but the whole thing is still being pressed down by the spring, but with quite a bit of tension and rather tight.
That shaved me around 10 degrees. From 78 to 68.
Also are you overclocking your CPU? If yes than how hard.