You know, you will probably need a vet's help to do this, but something to ask about. I have read ALL of James Herriot's books, many times. One of the stories I recall is a story he wrote about a dog he was treating who had the same symptoms as yours. The dog had enteritis, normally a simple thing to treat, but had progressed to where it wasn't responding to any treatment; constant vomiting, diarhea, shakes, etc. They didn't know what caused it either. What he finally did was to anaesthetize the dog enough to put her to sleep, (I mean natural sleep, not dead)...He kept her sleeping for 48 hours, or something like that...anyway, when she woke up, the vomiting and diarhea had stopped, and she did recover. His theory was that he was able to stop the stress and anxiety from all that violent bodily activity, and it gave the dog's body a chance to heal itself. He discovered this procedure accidentally on another animal. So, maybe a sedative would help your dog. Something to ask about...I would, anyway.