I read in a book for IBS that thin stool could be the result of spasms in your colon. The spasms contract the colon, thereby making the stool thinner than normally. There are certain herbs that can reduce the spasms. I believe, they include fennel, caraway seeds and anise. You can add them to your foods/salads. When you said that fiber makes the stool to thick to pass, were you talking about taking fiber products, or fiber through cereal? I find that fiber (from foods) and yogurt really helped me constipation. It's worth a try. Even if you are lactose intolerant, I read on this board several times that yogurt doesn't contain a lot of lactose, and, therefore, a lot of times lactose-intolerant people can eat it anyway. Be well.