My suggestion would be to pass different arguments to Wp_Query Class.
using ‘cat’ property, passing category Ids.
<?php
$custom_query = new WP_Query(
array(
'cat' => '1,2,3,4'
'post_type' => 'my_post_type',
'posts_per_page' => -1,
'orderby' => 'title',
'order' => 'ASC'
)
);
?>
OR
using ‘category_name’ property passing category slug’s as string parameters.
<?php
$custom_query = new WP_Query(
array(
'category_name' => 'category1+category2'
'post_type' => 'my_post_type',
'posts_per_page' => -1,
'orderby' => 'title',
'order' => 'ASC'
)
);
?>
More info about Wp_Query parameters:
http://codex.wordpress.org/Class_Reference/WP_Query#Category_Parameters
Hope it helps.
Good Luck.
Att