Is the slide changing happening on mouse hover alone or is the play interval between two slides set to a very short interval that you feel it's changing?Just check for that.
If it's happening due to time interval,
check out this heading "How to change the time interval between slides at runtime in the ASP.NET AJAX SlideShow Extender" in the link provided
http://www.dotnetcurry.com/ShowArticle.aspx?ID=287[
^]