Have you considered, instead of basing rank on upvotes/time, basing it on upvotes/views? Views could be either views of the actual link, or just views of the listing page containing the story, or a combination of the two. (Of course absolute number of votes would still have to factor in somewhat so brand new stories with one vote don't jump to the top.)

Perhaps this is already happening, but my (probably simplified) understanding of the system was that it only took into account time since the story was posted, not the number of views it's had, compared to votes.