I can't find the one I'm thinking of, which had the data very nicely graphed, but here's a related study:
http://www.journalism.org/resources/research/reports/debateeffect/positive%20versus%20negative.aspYou have to look at the .pdf to get the whole thing.
This study doesn't really look at positive/negative, it looks at Bush's positive vs. Kerry's positive (and some at the opposite). But the result-- that Fox just didn't give special treatment to Kerry, and in fact was not particularly positive towards Bush-- remains the same.