I know this seems like an odd forum for a post about tummy issues, but I remember at least one step-parent here talking about chronic stomach issues with their step-child and feeling like it might be stress-related. I ran across this article and thought it might be helpful. I can't find a postable link, but it is in the latest issue of Kiwi, which you can get to online at www.kiwimagonline.com and I think it will let you preview the latest issue.
If you'd rather google the study they refer to, you could google "Functional Abdominal Pain." The researcher's name is Dr. John Campo from Nationwide Children's Hospital in Ohio.
PM me if you want more specific info... I can't really remember if and how much of an article I can post without permission from somewhere (if someone else knows, I'm happy to type it in... it's less than a column long).