This sounds very similar to what I've experienced with Beau...but unfortunately I never figured out the cause for sure. It actually happened while he was under my mom's care for a few months, and it seemed to happen about once a month for a few months, only lasting for a couple of days at a time, and then he'd be back to normal.
He had diarreah, sometimes bloody, and was throwing up, mostly bile. My mom took him to the vet and they gave him fluids because he was dehydrated, and they also gave him some liquid medicine to help settle his stomach so he could eat without throwing up. And some pills for his diarreah. They took a fecal and other tests and found nothing. Next they did a blood panel, and again found nothing. They put him on Hill's I/d for awhile also, to help him eat. Since I wasn't there, I can't say for sure that it wasn't just something random that he ate. But the fact that it was recurrent with the same sypmtoms each time makes me think it must have been something else, not just something he ate. But I don't know what.
I did change his heartworm preventative, which *may* have been the cause, and which would make sense with the timing with it happening monthly. Maybe you can try using something different, if you think that might be the cause. Try and remember exactly when the sickness started each time and whether you had just given him anything like that.
I know there are some parasites that go undetected, so maybe he has something that just doesn't shown up. Or, the vet told my mom that if it kept happening (which it hasn't since then), that it might be pancreatitis (sp?).
Whatever it is, don't give up hope. Maybe you'll be able to notice some sort of pattern and pinpoint the cause.
Siggy courtesy of Tricia (hollysmommy)