I've always had this problem with Kwrite, which I use for all my editing. It still exists in my present configuration, which is Ubuntu 10.04, kernel 2.6.32-41, Gnome desktop 2.30.2.
The backwards typing seems to happen after a cut and paste operation. Restarting Kwrite gives me another half hour before it seems to happen again.
I also see the jumping cursor effect mentioned in this thread.