Error with "index.php?ag_customjs_var=js"?

A few days after regenerating cache, I get an error that this “remote file” can’t be reached http://www.mysite.com/index.php?ag_customjs_var=js

This happens every time, after a few days. It makes my site rather slow, so you really can tell when this error have occurred without even logging in. If you do, and regenerate cache – everything works as it should, until a few days later.

What can be done about it? It doesn’t reallt brake the page, it just makes it 3-4x slower.

http://ww.wp.xz.cn/plugins/w3-total-cache/