My mother's Siamese cat does this almost every week. Sometimes it seems to be after eating and then he tears around the house like a crazy loon and stops long enough to throw up on the livingroom carpet. Our times he was seen chewing on dried flower arrangements or live house plants. And then there are the hair balls. If a cat does it daily, like mentioned above it might be a digestive disorder or something more serious. If a vet is puzzled and has no answers, then it could be your cat is just stressed out from noises in the home or outside. Trees were cut down on my mother's street and the noise from the chain saws terrified her cat. He was hiding most of the time and throwing up the rest of the time. Not much help to you, but other cats do it too. 
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