LaTeX.org

I've got it work with such TeX code which I inserted right after the second part name:

\markboth{left}{Second Part Title}
\renewcommand*\thesection{\arabic{section}}

First line replaces left part of page header for second part. If I don't have it, I just see previous chapter's title.
Second line ...

It sims like a simple task, but I couldn't find a way to do this.
I have a book which has 2 parts. First part has 12 chapters and many sections, and everything looks fine as is. But second part has just hundreds of sections. I've put a chapter in before the sections of the second part, so the ...