I dont know if this exactly is causing all the problems you might be facing but i want to notify you one thing i have seen that you are doing wrong
<script src="Scripts/qTip.js" type="text/javascript">
function showToolTip(e) {
}
</script>
This usage is wrong. If you want that script block to function properly you should remove
src="Scripts/qTip.js"
from the script tag. Use seperate script tags
<script src="Scripts/qTip.js" type="text/javascript"></script>
<script type="text/javascript">
function showToolTip(e) {
}
</script>
If possible i will look more into the code and update this solution.
I tried to add qtip tooltips to a sample treeview and it worked like charm replacing the default tooltips with its own without much hassle. I used jquery to add tooltips to the treeview. Debugging your code and script seemed very hard( n need more resources like more of your code) as you seem to be doing a lot of other things at the same time as displaying tooltip.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script src="Scripts/jquery-1.3.2.min.js" type="text/javascript"></script>
<script src="Scripts/jquery.qtip-1.0.0-rc3.min.js" type="text/javascript"></script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TreeView ForeColor="Black" ID="TVContent" runat="server" ShowCheckBoxes="All"
ExpandDepth="0" CssClass="TVContentClass"><%----%>
<Nodes>
<asp:TreeNode ToolTip="Node 1" Text="Node1">
<asp:TreeNode ToolTip="Node 1 Level 2" Text="Node 1 Level 2"></asp:TreeNode>
</asp:TreeNode>
</Nodes>
<NodeStyle CssClass="NodeItem" /><%----%>
</asp:TreeView>
</div>
<script type="text/javascript">
$('div.TVContentClass a[title].NodeItem').qtip({ style: { name: 'cream', tip: true} })
</script>
</form>
</body>
</html>
This approach will replace the treeview's default tooltips with qtip ones. If it is custom html you want to show as tooltips instead of Node.ToolTip you will need some more changes. can help you if that's the case, but i will need more time.
Hope this helps.