glossy artifacts

Hello Blenderers!

I’ve ran into a strange problem. I have model sculpted with multires, and i painted a black and white texture to mix a diffuse+glossy and a glossy shader. It’s working so far, but there are some really strange artifacts on the glossy surface on the top.

Any ideas on how to fixit or what could be the problem or any other ways to make this?
Thanks in advance!