[Plugin: TOC ] Scrolling issue when using TOC with Fixed Widget

I am using TOC+ in the sidebar of our site with Fixed Widget.

Sometimes when one scrolls down the site, TOC+ stops in the middle when there is still text left in the bottom. And sometimes TOC+ scrolls on top of other content on the site (see image below).

Any thoughts on why this happens?

https://www.dropbox.com/s/q0axtukd245q6k2/Screen%20Shot%202014-11-26%20at%2013.48.12.png?dl=0