This is a problem, we have it with ours. Susi goes almost blind with rage when another dog comes towards us, its almost as if she cant see it and she barks and snaps in all directions. Snoopy goes into the attack but stops once the dog has gone.
We have been told by dog trainers that the only answer is far more contact with other dogs. We should go to a dog obedience class with a play-group. Our two have to be muzzled for this but we shall try it in the summer.
Another trainer swears by first teaching the dog to 'leave', i.e. go past food, toys etc. As soon as the dog goes to take the treat one shouts 'leave' and pulls the dog towards you. Once they have learnt that you can use the 'leave' command, when they go for a dog.
We are trying the following right now. Susi and Snoopy walk to heal quite well. If we see a dog coming we wave a treat in front of their noses and wait until the dog has gone past before giving it. They are supposed to then associate strange dogs with getting treats. Thats the theory!!!! It has worked a couple of times so far.
What did work was having a beagle and its owner come to afternoon tea. Maybe our two remembered her from their prison days, Jenny was also a research beagle, but they let her sniff about our garden without any trouble. If you have friends that dont mind risking having their dogs bitten maybe could invite them over and keep the dogs on a lead to start with.
Anyway, hope the above is some help