next_post_link order within category

Hi WP community,

I am using a theme called fotofolio to showcase a short film for a friend of mine.

I am having issues with the order that next_post_link() and previous_post_link is getting the post.

I need it to get all the posts from within a category (between 1 and 12) as they are then showcasing some photos for each category.
<div class=”alignleft”><?php previous_post_link(‘%link’, ‘Previous’, true, ‘6,15’); ?> </div>
<div class=”alignright”><?php next_post_link(‘%link’, ‘Next’, true, ‘6,15’); ?> </div>

Some categories are working perfectly, others – the posts are all over the shop, going from Category 3 to 8 or similar.
Is there another way to do this, using post_title as the order (I think it is using post_date)

Thoughts?

Your help is much appreciated.