My dog has something of the same problem. She is the loveliest thing around people, absolutely loves them. But she has a problem with other dogs. Not small dogs like our neighbor has. Although she has run over there when their idiot 2 year old started barking at her previously, she now ignores her when georgie does that. But the couple of times where she has run over, all she did was sniff while the other dog yelped like she was being killed. Which was kinda funny.
But last week, some lady was walking her poodle and Belle got a whiff of the other dog, ran over, and just attacked it. I got there right as she started since I was finishing up the caprice when she ran off from beside me and if she acts like she sees something then I usually tell her to calm down and keep a close eye on what she's doing until she loses interest. But this time she didn't even sniff the dog like she normally has in the past, she just ran up and attacked it. And whenever we take her somewhere in the car, which is all the time because she absolutely loves car rides, whenever she sees another dog she starts barking really scary like, almost like a viscous attack dog. Since it isn't aimed at us, it's pretty easy to get her to stop, but this worries us. None of our other Newfies have been like this around other dogs, the only difference is that this one is a rescue dog that we've had for a little over a year rather than the others which have all been bought from a breeder or from a woman who also had a cat (just the second one, the first and third were from the breeder).
*note: the other dog was fine from what I saw, not bleeding or anything, just scared out of its mind. and Belle got a few whacks on the nose as soon as I pulled her off and a few bad dogs as well. Also a swat to the rear as she went inside. She knew exactly what she did, that I have no doubt of seeing as how she's the smartest newf we've had so far.