A common problem which I'm sure there are plenty of sites out there with common solutions. So here's yet another someone might stumble across.
If you're CollapsiblePanel or Modal Popup flicker when the page is loading then it disappears. Or another way to describe it, the Panel shows while the page is loading then when the page is done loading it hides. Sometimes this process is very fast, causing a flicker sometimes it's slow, confusing the user.
You can fix that by setting a style on the Panel itself that is being hidden or collapsed.
<asp:Panel ID="pnlIDHere" runat="server" Height="0px" Style="overflow:hidden;">
That alone should fix it.