Hi Alex,
I'm wondering whether this is a bug in the web image viewer or a performance or rendering issue on the server. What browser are you using? Do you see the same thing if you try a different browser?
Could you try to show a 'console' for the browser to see if there are any error messages? I'm not sure how to do this for IE9 but for Firefox, you can add the "Firebug" plugin
http://getfirebug.com/, then run it and look under the 'Console' tab. This should tell you if you're getting errors retrieving the tiles.
You can also look in the server web logs /var/logs/OMEROweb.log to see if there's any errors there.
Many thanks,
Will.