Keith Driscoll on "wp-config keeps adding new lines"

Comment it our then make the file non writable. That really isn't a fix though since there is obviously some malware somewhere. Have you looked at your .htaccess file? Maybe you can use a plugin like duplicator to copy the site to your local machine and follow one of the many tutorials on fixing a hacked site. Here's one from Elegant Themes http://www.elegantthemes.com/blog/tips-tricks/what-to-do-when-your-wordpress-website-has-been-hacked