I have the exact same problem with yours. I tried Citrucel, and it sort of worked. It's a fiber supplement. Do not rely on the fiber in fruits and veggies. Citrucel has a special fiber. You may also try flax seed, which works for some people. Metamucil also works, but it made my heart race. But, these may still not be the complete cure, I should say.
I do not believe you have a stomach problem. Go, take an "upper GI" test, if you have health insurance. They will make you drink barium and have a look at your stomach and duodenum. Anything problematic in stomach and the beginning of upper intestines will show up. Just to make sure things...
My guess is that you have problems with liver and bile secretion. This is what I also suspect about myself. You may not be producing enough bile. How is your skin and complexion? Do you get easily tired? How is your mental agility and concentration?