complete code page selects with no cause

My Dreamweaver CS4 Student Edition is behaving oddly. Often when I am
working in code mode (PHP), most or all of the entire page will select
when I do no more than simply click a random spot in the page. The
only action I have found that will stop this (and only for the one
page I am working on at the time) is to enter a carraige return
somewhere in the page.

This seems like a bug in Dreamweaver. I've also considered the
possibility it is a memory issue. I so open some very large files in
Dreamweaver. Is there anyone who knows how to deal with this?