Why is PHP's register_globals option is enabled??? Thats rather
unsafe, any person has access to MediaWiki's source code, If they find
one mistake in the way vars are handled they could get your admin
password using an xss exploit?
As far as the page not working, what is the page you expect to work?
site.com/wiki/index.php
is error reporting turned on in php.ini if index.php exists it should
atleast throw some errors? The only other thing I can think of is did
you modify MonoBook.php? or index.php in some wierd way? I had added a
html comment under the </html> tag on my MonoBook.php file once and it
gave me simular trouble to what you have: sometimes only page headers
would be sent, no page content.
Chris