Keep Coding

I am not sure if this is a bug in Aquamacs (GNU Emacs 23.4.1) or what but it seems I can't something very simple: bring the cursor somewhere in the text by click, then press SHIFT and DOWN ARROW to select multiple line of text.
It just Aquamacs won't keep the selection starting where I placed the cursor but break it as new text appears in the window.

I had the similar issue with Macports Emacs. Maybe the following solution could be of help. It was because of a custom macro assigned to Shift-Down. It used to work perfectly on Linux but not on Mac. Though, when I restored default settings for Shift Down it worked out perfectly. To check if there is a custom macro assigned use a-x describe-key enter and press Shift Down. Make sure that there is the following response:
<down> (translated from <S-down>) runs the command next-line,
which is an interactive compiled Lisp function in 'simple.el'.
If there is a different response it could be a custom macro assigned with global-set-key (kbd "S-<down>") somewhere in configs - and it should to be removed.