Position moves when page scrolls

Hi guys, here’s a link to one of my post for you to see the position changes in action. http://flme.ch/ua7

When the bar is at the top, continue scrolling down to notice that the buttons are jerking.

I’ve checked the CSS and noted the “top” and “left” values are constantly changing. This is the cause to the jerkiness.

I’ve checked other themes as well, and while this does not occur on some themes, on other themes it did occur as well.

Please kindly look into it. Thanks!