Have you tried probiotics? The two most tested strains are Saccharomyces boulardii and Lactobacillis GG (LGG). They are available under brand names Florastor and Culturelle. (The former is also available at Swanson Vitamins under the scientific name—Sach etc.) I’ve had success using Bacterium longum BB536 (Life Extension Bifido GI Balance) for IBS. I researched these strains by reading medical journal articles on PubMed.