Random Post Order in Montezuma

Hello,

I am trying to randomize post order in montezuma theme. I tried placing this line in function bfa_loop of template_tags.php as the first line

query_posts($query_string . '&orderby=rand');

This randomizes the post order on all pages including category pages. However, once this line is here, the category filter stops working e.g. selecting a single category would show posts from all categories including posts that do not belong to selected categories at all.

What is the recommended way to achieve random post order in montezuma while still keeping the category filters working?

Thanks