I've said this before, but I'm believing it even more these days...I think spontaneous vomiting usually has a fairly definite cause. Some people, however, do get chronic nausea with IBS and a cause may not be pinpointed easily, but attacks of vomiting, in my opinion, warrant a few tests: endoscopy (which you are getting in 3 weeks, that's good), abdominal ultrasound, and possibly an abdominal CT (to check for inflammation in the small and large intestine) and a HIDA scan (which checks the gallbladder's ejection fraction).
Vomiting is miserable. I hope the tests yield some results for you. Keep us updated or let us know if you have more questions.
When I was talking about spontaneous vomiting, I meant sudden attacks that are not attributable to something, say, like an enteric bacterial infection (which I'd guess causes most episodes of vomiting, at least in the general public - most people call it a "stomach flu," although technically it's gastroenteritis caused by a number of different of viruses, and they usually just go away on their own). I worry about people who have severe vomiting that comes and goes in spurts because that sounds like more than "just" gastroenteritis.
If you've had a negative upper and lower scope, a small-bowel follow-through is a good idea, and an abdominal CT scan and possibly a capsule endoscopy (where you swallow a pill-sized camera and it takes pictures as it travels through the digestive tract) might be warranted. If you have inflammatory bowel disease in the family, that's all the more reason to get a more thorough work-up, especially since you also experience diarrhea. Have you had any blood work done recently?
I have been through a lot of testing in the past because of severe, intractable diarrhea, so a lot of my info is anecdotal, but I'm also a medical student. My knowledge is still pretty limited, though, so don't take my word for anything