Session Expire Problem

Dear all, I built web application with C# and SQL Server. The problem is that when I run the web application for a certain time say about 5 minutes and click the drop down list. The page stop loading and all other hyperlink inside the application was killed. The session tag in web.config is the default value and I have set cookie value expire after logout. I don't know what happen to session. Could anyone tell me the problem? Waiting for reply!! Thanks!! Attach code for login success page and web.config

/*
* GetSessionValue() is to read session value which is already set in Login page
* */
private void SetSessionValue()
{
	if (Session["sid"]!=null)
	{
		//read staffID and staffPcode session value
		lblStaffId.Text = Session["sid"].ToString();
		lblStaffPcode.Text = Session["pcode"].ToString();
	}
	else
	{
	Server.Transfer("//SMS/LoginUser/Login.aspx");
	}


}


/*
* SignOutUser() is to perform session clearance and cookies expiration
* when user click logout
* */
private void SignOutUser()
{
	Session.Clear();
	Session.Abandon();
	Response.Cookies["myCookie"].Expires = DateTime.Now;
	Server.Transfer("//SMS/LoginUser/Logout.aspx");
}


/*
 * btnLogout_Click() is to perform basic logout operation when user click
* */
private void btnLogout_Click(object sender, System.EventArgs e)
{
	SignOutUser();
}

web.config