This sounds very similar to what I went through with Prince. It seemed to start just out of the blue and for no apparent reason. He wasn't eating different food or on any new mediations or anything.
Prince used to throw up a lot - multiple times some days. Sometimes he threw up when he first got up in the morning, sometimes it was in the middle of the day and sometimes right after he ate.
Prince almost always acted normal just beforeand just after throwing up. Sometimes he would eat it again - GROSS!
The vet said he was probably allergic to something in his food. He was put on a prescription food. It seemed to help a little but not much.
The vet said a dog's stomach is designed to purge anything that is upsetting it. He said it was probably nothing to worry about but he told me to take Prince into the office as soon as it happened if it was possible. He wanted to take his temperature and see how he was acting as close to the episode as possible. We made 5 trips to the vet in one week - that was the week Prince bit the vet!
Apparently he got tired of the thermometer being stuck "there".
I switched the dogs over to raw and home cooked meals almost 3 months ago. He has only thrown up twice since then.
After all of the research my Mom and I have on dog food and the improved health of her dogs since switching to raw and homecooking, I'm convinced it was the cr*p in the dog food that was making him throw up. Even Angel has more energy and spunk (I prefer to call it attitude) since switching.