Problem with window.open and form, help me please :(

Hello, i have a little problem; i have a form so defined:

<form id="form1" method="post" action="mypage.php" target="mytarget"
onsubmit="return false;">
<input type="submit" name="join" value="Click
Here!"
onclick="window.open('about:blank', this.form.target,
'width=320,height=240,resizable=no,
toolbar=no,location=no,directories=no,menubar=no') ;
setTimeout ("this.form.submit()", 10000); " />
</form>

The problem is that i click on "Click Here!" a new window is opened, but not
every content of "mypage.php" is in this window opened, in sense that
sometime is in window "mytarget" and othertime in browser, and page of
"mytarget" window remain blank.
As i can solve this problem, in mode that ALL TIME content of "mypage.php"
is put in "mytarget" window? and not in browser?
Thanks, for all and Good Christmas.
Cooper: