Hi there,
i suggest you change the frame rate for that particular scene. (by default it is set to 12) or if possiable, extend your movie over more frames.
In order to center your swf in a browser place the <center> tags before your <object> and <embed> tags in the HTML. Don't forget to close it with the </center> tag after the <object> and <embed>.

Cheers,
micmac