I agree with the poster who stated you should videotape your daughter the next time she has an episode, so the doctor(s) at Mayo and Children's Hospital can see her distress and know that it isn't because you're an overanxious mother.
If, however, you take your daughter back and they still can't figure out what's wrong--think about taking her to a Naturopathic doctor, preferrably one who specializes in children. It will be more expensive, granted, but sometimes, they can help when allopathic medicine fails. Especially because they're looking at the whole person and not for a specific disease. And I've found that they "listen" to you a lot more than a doctor does.
If your daughter is still displaying such harsh measures of pain (banging/thrashing head, thrashing body) and she hasn't eaten solid foods for 6 days--there is definitely a problem there. But until you either have your daughter at the doctors when she's displayed the symptoms or you catch them on video, the doctors won't take you seriously. And I don't think intussessucession (sp), either, because if she hasn't been eating solids for 6 days, then there isn't anything in the colon for the colon to move/work on the food--so the bowel shouldn't be "eating itself up" or "going inside itself." That's what I personally think, but I don't know how the bowels work. If, however, there's already dead tissue in there...then yes, she would be having major pain. And I don't think the "dead tissue" stops once it starts, either. It (the diseases/dead bowel) has to be removed.
If you have not read the article on intussusception on Wikipedia, I urge you to. The recommendation is to do an ultrasound as well as an x-ray. If they haven't done an ultrasound of the entire bowel area, I would seriously raise the roof and ask to have it done. While the article does state that about 80% of the intussusceptions will diminish on their own, they still need a little help from the doctors and a few interventions (like barium or sodium enema), but if it can't be resolved or the intestine is damaged, then surgery is called for.
I hope you get it figured out, so the issue gets resolved and your daughter isn't living with so much pain.