If your IBS symptoms are primarily diarhea (rather than constipation, or alternating) and gas, and abdominal pain, do a seach on google for IBS and Fructose intolerance. That's what mine turned out to be (after 10 years of suffering). After eliminating fructose from my diet (MUCH harder than it sounds) I have no more symptoms at all. It's worth looking into if your symptoms fit.
Maggie