Check on acid reflux. My daughter used to throw up quite frequently, finally we had an upper GI done and sure enough she had severe acid reflux. Now that she's on her medication she only throws up if we miss a dose.
Mystery Diagnosis finds people with extremely rare and strange cases. I've seen two where children throw up frequently. In one, the child's stomach was in his chest and the other the child was having nocturnal seizures that manifested in vomiting. Odds are it isn't those. = )
My daughter did. She always seemed to find the worst possible place for it too. Restaurants, an airplane, the car, you name it, she puked there. She did outgrow it when she was about 4. No doctor ever said this, but I think she had acid reflux. She's fine now.