[Lynne]

Not the showthread template, the showthread.php page. That is when your tabs are broken. So, go through showthread and add the files one by one and see if you can get your test page to 'break'. Like:
require_once(DIR . '/includes/functions_bigthree.php');
require_once(DIR . '/includes/class_postbit.php');

Add those at the top under
require_once('./global.php');

See if you then get the funny url. You need to find out what is making the url go weird and the only way to do it is to add this stuff one by one. Add in all the hook lines so that it includes your hacks and see if that breaks it. Just keep adding stuff from the showthread.php page into your test.php page until it breaks.