Jquery script issue

Jquery script issue

I'm pretty new with jQuery.
Trying to write a simple script to slide up and slide down.
<script type="text/javascript">
$(document).ready(function () {
if ($('header').is(':visible')) {
$('header').hide(function () {
$(".masterhead").mouseover(function () {
$("header").slideDown('slow');
});
});
} else {
$('.masterhead').mouseout(function () {
$('header').slideUp('slow');
});
}
});
</script>
The problem is when I'm trying to see it in the browser, it keeps sliding
up and down not the correct beahvior i was hoping, I tried with
SlideToggle but the same thing happens. I tried a lot of solutions from
this site but the same thing.
I will appreciate any help with this.