When you are using a asp.net menu when any postback occurs and the page gets rendered again the menu "flickers" for a moment. When this happens the whole menu items will be displayed for some time.
As following post suggests setting display:none in #menu ul li ul solved the menu flicker issue for me .
http://stackoverflow.com/questions/3240873/how-to-get-rid-of-ugly-aspmenu-flickering
Edit
If the above solution doesn't work try setting the RenderingMode ="Table" in the Menu .
Cheers
samitha
As following post suggests setting display:none in #menu ul li ul solved the menu flicker issue for me .
http://stackoverflow.com/questions/3240873/how-to-get-rid-of-ugly-aspmenu-flickering
Edit
If the above solution doesn't work try setting the RenderingMode ="Table" in the Menu .
Cheers
samitha