The problem is in the way that Internet Explorer 6 renders its layered elements. A DIV that has nothing behind it on the page is rendered bit by bit (for efficiency) as the page scrolls. If the scrolling is anything other than 100% smooth the rendering fails leaving the sorts of glitches shown above.
The simple solution to this is put something behind the DIV(s) that are causing the problem. When the DIV(s) have something behind them IE appears to change the way in which it renders them as the page scrolls. Now IE will re-render the entire DIV as the page scrolls, less efficient but it guarantees the entire DIV is displayed correctly. [...]