Erm... this could be my story "Severus Snape, favourite enemy". Look it up at: http://hp.adult-fanfiction.org/story.php?no=600000287 (maybe changing the section wasn't such a good idea )
There are more James/Severus-stories out there... like this:
http://hp.adult-fanfiction.org/story.php?no=544179869 (pure sweetness!)
Or look up this members profile: http://hp.adult-fanfiction.org/authors.php?no=1296832570 for more James/Severus-stories, all of them are pretty good (and kinky ), for example Lying together and Expectations change. Flawless writing style.
Hope I could be of any help, have fun!
BlackPriestess