Has anyone ever done a comprehensive blood screening to see if you have a vitamin deficiency? Chronic diarrhea and IBS, could be caused by a lack of vitamin D, A, E, K, Zinc, Iron or calcium. At the very least, the frequent bowel movements would make you more likely to have some form of vitamin deficiency since your body may not be absorbing the nutrients.
Also, when you say you gave up all the supplements as a test – does that include energy drinks, and things like Vitamin Water? I had frequent diarrhea episodes for years – until I figured out it had something to do with a combination of caffeine, and citric and malic acid that was in many of the beverages I consumed.
Lastly, I assume you were taking a good probiotic? This will help alleviate some of the symptoms as well as anything that helps reduce information such as fish oil, COQ10, etc.