Have you taken EMMA to the vet? When Ted was sick, toward the end, with his cancer (not saying Emma has something like that) we had a lot of random attacks from Bear who was always buddy buddy with Ted before. But if there's something that's changing Emma's chemistry, how she smells, etc, it might be causing Jazz to react badly. Just another thought.
<3
I would also be looking closer at Emma - I've seen some dogs (in other households, so no personal experience here) that would attack a weaker/sicker/elderly dogs unprovoked; somehow a "survival of the fittest" if you will. Maybe Emma is having the beginnings of an Addisonian crisis and Jazz senses the weakness? Wouldn't hurt to check her levels if she hasn't been in recently.
I know that most of this has already been covered and I may be repeating a little, but I had some similar thoughts to the above quotes. That maybe it has something to do with Emma's condition being unstable with the Addisons?
I was dog-sitting a pair of keeshonds overnight this weekend. One is epileptic and prone to seizures and he is on daily medication. Normally, the two dogs are buddies with each other. However, the night I was there, the other dog woke up at 3am and instantly attacked the epileptic one, whom was asleep and completely unprovoking. I mean REALLY attacked, too. I managed to get them apart without any real harm being done, and I had to keep them separated the rest of the night and the next morning because the one would NOT leave the other alone. Even if he saw the other through a baby gate he went ballistic trying to get through to him.
Sure enough, 12 hours later the epileptic one (the one being attacked) had a seizure. My friend (who was dog-sitting the next night) was there when it happened, and once it was over and the meds were all given, the other dog went back to normal as well. The owners were just as shocked as we were, because this dog had NEVER reacted that way with the seizures before.
I realize that story may not help out as much... but I hope it does. I agree with Redyre that it's probably best to keep them separated all the time now, at least until you find out what is really going on. But like the others ahead of me said; in addition to keeping an eye on Jazz, keep an eye on Emma, too. There might be something going on with her that Jazz is picking up on that you can't see yet.
I hope it gets better soon! Sending some good vibes your way!