Syndrome is most commonly caused by stones/sludge in the biliary tree or stomach acid related conditions like gastritis, peptic ulcer disease
and acid reflux
. While some people have psychosomatic symptoms, I agree that this is less likely. When you say she has been fully tested you need to make sure that includes upper endoscopy
of the esophagus, stomach and duodenum. I would be interested to know more about exactly what time of year the attacks happen and whether you can identify a trigger - feasting, fasting, alcohol, constipation
etc... Would be happy to continue the advice once I know a little more.