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Thank you all so much for your responses.
Here's a quick summary of the responses I got.
A lot of people told me I should switch doctors...my doctor is actually
very open-minded, and when I went to her today, carting all my printed-out
responses and a few notes to boot, she listened very intently to what I
had researched.  She then referred me to a GI specialist.  Now, I guess
I'll have to see how open-minded the GI is.  I cancelled my barium x-ray
in favor of the sigmoidoscopy (I don't like the idea of the toxic barium
in my gut, and niether did a few respondants), so we'll see what the GI
has to say.
Two people suggested parasites were a problem, one of whom had every single
symptom I had and is currently undergoing a course of Flagyl (kills Giardia),
and the other whose tests are still being run to determine the problem.
One person suggested looking into cystic fibrosis.  Her daughter had CD-type
symptoms, but the doc tested her for CF, and came up positive.
After getting all the responses, I was talking to my father and he told me
that he had a lot of trouble making it to the bathroom on time before he
stopped eating wheat (he stopped because wheat irritates his arthritis), he
also mentioned that my grandmother told him that she had "never had a solid
bowel movement in her life".  My aunt (dad's sis) recently had to have part
of her colon removed because it was leaking toxins into her body, and my cousin
(dad's sister's son) is having bloody mucous in his stool as well.
I was very interested to find all this out...seems to point to something
genetic...ugh.  We'll see what the GI says.
Thank you all so much for your support!
Jill in CA