Next post with a given tag

Hi there,

I currently use wordpress 2.5 as a photoblog and a portfolio. Basically my portfolio functions in exactly the same way as the blog except that the photos are tagged as portfolio. Unfortunately when I view a single page however the next and previous links go to the next and previous post chronologically and not per tag. Does anybody know how to make the next and previous links be next and previous with a given tag?

Thanks in advance,
Alex

ps. you can see my blog here to give you a better idea of what I mean.