
It sounds like she needs to become more confident around strange dogs. she behaves badly when she has the back up of her pals, because she is frightened, but on her own is too afraid to put on a 'clear off' threat display and opts for making herself inconspicuous.
I think lots of socialisation with laid back sensible dogs would be most helpful.
At shows so you tend to only keep her on the bench/cage and among her own breed? Entering variety classes would help her as she would be among other dogs in a controlled way, building her confidence.