Archive filtering

You need a tax_query argument to WP_Query, if I am reading your question correctly.

$my_query = new WP_Query(
        array(
            'tax_query' => array(
        array(
        'taxonomy' => 'category',
        'field' => 'id',
        'terms' => array($featured_ID,$curr_cat->cat_ID),
                    'operator' => 'AND'
        )
         )
        ),
        'posts_per_page' =>8
    )
);

Untested, but I think that should be close.

Also, where does global $cat; come from? I don’t remember seeing that as a Core global but maybe I’m wrong. If that isn’t set your code will produce a very broken argument:

'category_name=featured,No Category Found!!&showposts=8'

That may be some of the problem you are seeing.