header acting up - cut in half

[tat1973]

when i scorll down and back up again, my header is cut in half -_-
also, if i alt-tab to other browser and alt-tab to come back again to my site, my header is cut in half....
what am i doing wrong?

my /* HEADER */ section in the stylesheet look like this:
/* HEADER */
#navMainWrapper {
}

#navMainWrapper a:link , #navMainWrapper a:visited {
}

#navMainWrapper a:hover {
text-decoration:underline;
}

#headerContent {
background:#1a7fb7 url('../images/logo.gif') left left no-repeat;
padding:50px;
}

#nameWrapper {
padding:10px 0 10px 30px;
}

#nameWrapper a, #nameWrapper a:visited {
display:none;
}

#taglineWrapper {
font-size:20px;
padding:10px 0 10px 60px;
margin-top:-30px;
color:#fff;
}

#navEZPagesTop {
font-weight: bold;
text-align:center;
font-size:1.2em;
}

#navEZPagesTop a {
color:#fff;
}



.navMainSearch {
margin:0 10px 0 0;
line-height:0;
float:right;
}