I had similar script errors in one installation. It was related to the php4 binary. Just as a test, please try to install php5-cgi (you dont have to remove mod_php5 for this) and then replace the line:
AddHandler php-script .php4
Action php-script /cgi-bin/php4-cgi
AddHandler php-script .php5
Action php-script /cgi-bin/php5-cgi
in your configuration and test if .php5 files work as cgi.