You are right, no issue with the demo (Opera).
There is just one other plugin on the page what could cause this issue: “Meteor Slides” (using jQuery).
It seems the pagescroll is preventing the browser-scroll multiple times, every time it flickers/scrolls down a bit and back to the top (many times in just one second). After that it scrolls down to the id. Sometimes flickering while scrolling, sometimes not.
It′s a combination of two plugins wich combined causing this issue.
Great support by the way. I never thought to get an answer that fast.