Keyboard navigation conflict with another script

HI, thanks for the plugin, I’ve been using it on many WP installs, jsut recently I noticed a small problem with one of my sites where the plugin is implemented.

I use this function to enable keyboard navigation between post.

(function($){
   $(document).ready(function () {
      $(document).keydown(function(e) {
         var url = false;
         if (e.which == 37) {  // Left arrow key code
            url = $('.prev a').attr('href');    }
         else if (e.which == 39) {  // Right arrow key code
            url = $('.next a').attr('href');    }
         if (url) { window.location = url;   }
     });
   });
})(jQuery);

But it stops workign as soon as an image is clicked and the plugin takes over.
How can I fix it?

Thanks

http://ww.wp.xz.cn/plugins/wp-jquery-lightbox/