UPDATE: This turned out to be a bug in R14 of the SDK tools. It has been fixed in R15 which was released on Oct/27/2013. Updating to the latest release solves
I also think it is "more aggressive" since the latest update! And it actually stops auto-scrolling when I scroll 10 or more lines up.
The best solution I have is clicking in the line you are interested in. If the logcat scrolls down too far and you don't see your line any more just press ArrowUp or ArrowDown to jump back to the line you were looking at.