I don't know if this will help but in /var/log/messages I find a load of these. An entry per crawl or page found:
preg_match(): Unknown modifier '|' in /home/httpd/htdocs/sitemap/pages/class.grab.inc.php(2)
BTW the only config thing I can think of that would cause this sort of problem to your script would be that safe mode is on. But that said, that should just mean that your script cannot set a max exec time.