To be honest, I've never really read up too much on IBS. But since I read your post, I got to thinking about myself. Now that I think about it, foods and drinks with HFCS really upset my stomach, it usually results in diarrhea.
I think you mentioned something about artificial sweetners? I love iced coffee, but coffee in general upsets my stomach, especially when I would order one with splenda instead of sugar to avoid taking in too much sugar.