@osisposis - You might try regular soda water, unflavored, to help if you want to avoid the chemicals in diet pepsi. Also, lemon juice in water was very good for helping me burp when my stomach acid was low.
Have you tried taking betaine HCL with pepsin (here's what I take:
http://www.swansonvitamins.com/swanson-premium-hydrochloric-acid-pepsin-250-caps). Stomach acid is important for keeping your gut bacteria in balance, so it might help your diarrhea. Low stomach acid contributes to or can even cause SIBO, besides causing poor absorption of food and nutrients.
A doctor told me to start with one capsule of the HCL before a meal and work up to where I felt "acidy" and then to back off by one. My "acidy" point was 4 capsules. It's weird, when I felt it, I knew what he was talking about.
And I read somewhere that the pepsin was important as well, though I can't tell you why, but it's better than plain HCL by itself.