Ditto what Marko suggested!
We also went through this with our first setter. It's very stressful, but hang in there.
Obedience training is a must. Giving your dog a job might also be helpful--if there are agility or tracking programs near you and you have the time to invest, sign up! Exercises both the mind and the body, and a tuckered dog is always a good thing.
If the behavior persists, start looking around for a good behaviorist. Your vet might be able to give you some recommendations, but do some homework and check out any names. (Not all behaviorists are equal--you're looking for one with a proven track record on aggression and who uses methods that you agree with.) Ask lots of questions before making your decision.
Please keep us posted on how you get on.