Close Page in Frame control, NavigationService

Hello, 

I use a frame in a Window:  

<Frame x:Name="mainFrame"  NavigationUIVisibility="Hidden" />

and many Pages...

To change page from Window i use:

nameFrame.Navigate(new A_Page());

And to change page from another page:

NavigationService.Navigate(new A_Page()); 

Problem is:

For example i open page A, where i Start a timer. After a time i open page B, and timer from A still works. Why it still works? How can i stop it if i change Frame source from Window, not from Page? What should i do such that when i navigate to new Page, to close current Page? 

Pls Help