Problem in navigation between pages in wp8

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C#

Windows-Phone-8

Hi every body
I'm building an windows phone8 app which have several pages.I use Navigation code for back button in each page.

private void Back_Tap(object sender, System.Windows.Input.GestureEventArgs e)
        {

            NavigationService.Navigate(new Uri("/Menu.xaml", UriKind.Relative));
        }

if I open 2 pages and back to main menu by back button, when I want to go to new page it has error "unhandeled exception":
in App.xaml.cs page:

// Code to execute on Unhandled Exceptions
        private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
        {
            if (Debugger.IsAttached)
            {
                // An unhandled exception has occurred; break into the debugger
                Debugger.Break(); // it is where my app stop from debugging
            }
        }

but if I open each page one by one and exit app and again go to second page and exite an again do it for other pages,every thing works correctly.
I think it's because of stack that became full and it happend so I try to find a way that on back_Tap code, after navigating I delete stack but I couldn't find out.
it's just a guess...
now , Is there anyone who knows what is my problem and how can I solve it?
thank you for spending time on my article...