There is a hidden folder in user's home, .kde. In there is another folder, autostart and a file, "sorticons". Delete that file (or if you are worried like I was the first couple of installs ) rename it. That drove me absolutely batty for a couple of days.

The icons will stop rearranging every login. The desktop will also come up at lot faster (in my case it would "sorticons" for ages on my slower system )