web links = Warning: Cannot modify header information

Warning-headers already sent in Z:\www\phpfusion7005\maincore.php on line 319.
1. safe mode=on
2. ver 7.0.0.5
3. no changes made to maincore.php
4. recently added new page in root directory...worldphotos.php.
page works OK no errors..
Problem only when working web links WITH navigation pages..

<?php
/*-------------------------------------------------------+
| PHPFusion Content Management System
| Copyright © 2002 - 2008 Nick Jones
| http://www.php-fusion.co.uk/
+--------------------------------------------------------+
| Filename: World Photo Search
| CVS Version: 1.00
| Author: John De Sousa
+--------------------------------------------------------+
| This program is released as free software under the
| Affero GPL license. You can redistribute it and/or
| modify it under the terms of this license which you
| can read by viewing the included agpl.txt or online
| at www.gnu.org/licenses/agpl.html. Removal of this
| copyright header is strictly prohibited without
| written permission from the original author(s).
+--------------------------------------------------------*/
require_once "maincore.php";
require_once THEMES."templates/header.php";
if (!iMEMBER) { redirect("index.php"); }
srand((float) microtime() * 10000000);
$inputTags = array("Cape Town",
"South Africa",
"Table Mountain",
"Durban",
"Plettenburg Bay",
"Cape Point",
"Chicago",
"London",
"RWC 2007",
"Cardiff",
"Wales",
"Melbourne",
"Australia",
"New Zealand",
"Madeira",
"Funchal",
);
$rand_keys = array_rand($inputTags, 2);
if (!isset($_POST['submit']))
{
$FlickrTag = $inputTags[$rand_keys[0]] . "\n"; }else{
$TagName = $_POST["TagSearch"];
$FlickrTag = $TagName;
}
opentable("World Photo Search");
echo "<br /><center>
<table border='0' width='50%' id='table1' cellspacing='1'>
<tr><td width='50%'>
<form method='post' action='worldphotos.php'>
Try Your Own World Photo Search::<input type='text' class='textbox' value='$FlickrTag' size='35' maxlength='35' name='TagSearch'>
<input type='submit' class='button' value='Search' name='submit'></form><br>
<iframe align='center'
src='http://www.flickr.com/slideShow/index.gne?group_id=&user_id=&set_id=&tags=$FlickrTag&favorites=own' frameBorder='0' width='520' height='400' scrolling='no'>
</iframe>
</td>
</tr>
</table>
<br />";
closetable();
require_once THEMES."templates/footer.php";
?>