The $post->ID is not correct on some pages

I have added the following code to the beginning of my header.php file so I can redirect the page if the ‘direct_jump’ metapost data value is set. That part works (with the print and header statements commented appropriatly), but I have found that often the $post->ID value is incorrect, and it will jump to the redirect page when it is not supposed to. It looks like the $post array is not getting cleared or set correctly when I go to some pages, including the home page and the main category page of the posts. Any ideas on how to proceed?
— $thisPost = $post -> ID;
— $gotoURL = ”;
— $gotoURL = get_post_meta($thisPost, ‘direct_jump’, true);
— if ($gotoURL != ”) {
— print (“thisPost = $thisPost, “);
— print (“header(‘Location: $gotoURL’);”);
— //header(‘Location: ‘.$gotoURL);
— }