Portfolio Grid: next and previous for only relevant category

LOVE your theme! Thank you! :)

1). I created a page to showcase all website designs; http://nityiadesign.com/Sandbox/website-design/

2). The problem is – After you click on that item for a closer look and then hit the next or previous arrow — I am taken to the project that was added before or after as a post — and NOT to next or previous ‘website design’ category item as displayed in the grid.

3). How can I ensure that the the next and back will only roll through the relevant category- here is another example:

http://nityiadesign.com/Sandbox/portfolio-item/product-photography/ I want to showcase only product photography items.

Thanks for your help!