.htaccess max_execution_time
You can alter the index.php file and add [set_time_limit][1](300); anywhere below the &lt;?php tag. 

You can also try to set it by ini_set('max_execution_time', 300);

For the reason why it returns a 500 Internal Server Error, the server does not support the php_value and php_flag parameters through the htaccess file or it doesnt use the php5 module. You can try setting the php_value directly and check again for errors, or contact your hosting provider for support