Using the latest JDK nightly (with some scroll pain fixes), if you drag the thumb down while a page is still loading, the scroll bar can get into a state where it rapidly oscillates between two or more locations. There are other situations where this problem occurs, but it's most repeatable via dragging the thumb down during loading. The only way to get out of "wild oscillation mode" is to drag the thumb to the top of its range, and hold it there while the need to oscillate subsides.

Try using ftp://prep.ai.mit.edu/pub/gnu. You may need to drag the thumb down a few times, and drag it around a bit, but once this effect is started, it's hard to stop.