All,

It looks like this was one of those subtle 'null' errors. Since I have changed the following code from

if(Session["USERID"] == null)
{
    Response.Redirect("Logon.aspx?ReturnTo=" + PageName + ".aspx");
}

to

if(common.IsNull(Session["USERID"]))
{
    Response.Redirect("Logon.aspx?ReturnTo=" + PageName + ".aspx");
}

My spurious redirects have disappeared. It seems a little weird to me - sometimes the compiler warns you you cannot compare to null, other times it lets the comparision slide right on by.

Thanks to everyoen for the suggestions - just another part of the learning curve.

Doug