[Greasemonkey] Function oing out of scope?

[Daniel Sheets]

function playThrough()
{
  unsafeWindow.ShowForward();
  timer = setTimeout("playThrough()", 1000);
}

I get an error saying "playThrough is not defined". I'm assuming that its
going out of scope before the next timer event.
How can I keep it from doing that? I need this loop to go on and on until I
stop it.

Thanks!