Recently working on a list box I saw unusual problem. On this page I was calling couple of function in java script using mouse rollover event. Every time these events were triggered, the list box used to flicker.
Even after setting exact height for table/td and list box, flickering continued. The only way to deal with this, at least in my case was to set “Row” property of list box.
<asp:ListBox ID="lstExample" runat="server" Rows="6" Width="200" SelectionMode="Multiple"></asp:ListBox>
Hope this helps.