This one is difficult isn't it, sounds like the usual tests have been done, antibiotics given and still no progress. Are you planning on going to see a specialist or get a second opinion? Might be worth considering. I presume your vet has already tried Buscopan as this is very good for stopping the stomach cramps and the diarrhoea.
Now first may I say I'm not going to explain this very well but it is something I found out a while ago from my breeder and from past experience it appears to be correct. Please feel free to laugh at my terminology

When you dog is passing diarrhoea does she pass it in the normal way and the stools are soft/runny or does it come out like an explosion ?? Is there ever any blood in it? Usually if it comes out like an explosion then this normally means the upset/infection is in the bowel, if she passes normally then it usually means it is coming from the stomach. Either way unfortunately I can't offer you an explanation but just thought it was something else to think about.
I hope you get the problem sorted soon, do keep us up to date with any further findings/progress.
Lots of luck and give her a big hug from me. She must be fed up by now.
Claire.