I have a little over a 1 yr old Dobie, had him a year now. Got him from the Humane Society. He was turned in because the resident Rottie didn't like him. We have done dog obidence 2 and puppy kindergarten, taken to the pet stores, dog parks, plays with the neighbors dog and kids. He was WONDERFUL in puppy kindergarten with both big dogs (Wiemenmer puppy) and little dogs (pugs). We had absolutly no issues and he was learning well.
Went to the same trainers for obidence 2, same location, and even had some of the same dogs from puppy class. During this time we continued to take him to the dog park too. Well half way through our classes he began to have an issue, you don't know when it will come out or if it even will. He would occassionally turn on the dog next to him, both male and female, snarling and lunging. The dog he was trying to attack would have done nothing. Sometimes they would react back but generally not. You'd get him under control and he'd be fine. He could go 1,2 or so classes and be fine and then he'd pick a different dog to have an issue with. Now he's started to do it at the dog park. We now muzzel him so he's not a danger to anyone but he still lunges and snarles under the muzzle and is a pain in the butt. The age of the dog doesn't matter either, generally it is the same age or older but the last time he went at a puppy. He has never had an issue with the neighbir dog tho. At the dog park it is not everytime we go that he has an issue and when he does git is only one dog he ever has an issue with. Once that dog is gone he is fine. He's never snarled at kids or humans, it seems to be isolated to dogs. He's also never snarled or gone after little dogs such as pugs, it's been labs, weimenmiers, Large collies, etc.he has never actually connected with another dog, just snarls and lunges (if on the leash) or chases em if off the leash, so he has not actually hurt anyone (yet).

Any ideas on how we can solve this?

Added info: Have tried choke chains, getting agentle leader tonight, tried short and long leashes, regular collars too.

how do you react when he does this? what do you say and how do you control the situation?

what is his body language doing, are his ears back, does he grumble? bark? or does his lip curl all the way up as he is doing so? what position is his tail in?

will be able to advise a  little more when I have more info.

In the meantime, when he does this ignore him whilst making sure he doesnt get too close, then as soon as he is quiet reward him. See how that goes. It will take quite a few attempts.

spitzed x

If we see a dog coming I bring the dog in closer, generally grab the collar for extra conrol. I get out a treat and try to distract him with a treat under his nose, asking him to sit. I try to also get between him and the other dog. I call his name to get him looking at me and continue to talk to him. If he stays seated telling he's a good dog and giving him treats.

If he sees the dog first and has gone into psycho mode I get to his collar and say his name to focus on me. I also tell him to keep walking and "pull" him along.

I'll have to watch him more closely next time for ears and tail. He does snarl his lip all the way back, he is lunging forward.Sometimes his shackles are raised sometimes not.

I'd suggest that by tightening the lead and holding his collar you are sending him the message that you are concerned about the approaching dog.  It may be worth considering a muzzle for him while you retrain him (with help from a behaviourist perhaps) to 'watch' you.  the muzzle will take away any anxiety you may have that he will do the other dog some damage.

I'd start by find out how close he has to get to another dog without reacting, and try and keep him outside of that zone to start with, giving plenty of rewards for ignoring other dogs.  ALWAYS keeping the lead loose........  Once he starts to realise *why* he's being rewarded, you can start to decrease the distance - always watching him for signs of stress - drawing back of the lips (like a 'smile'), narrowing of the eyes.  At this point ask for the 'watch me' command (which you've already trained and used in other places by showing him a treat close to his nose, then drawing it away to hold it under your chin, reward him for making eye contact with you - repeat this until you can make it a hand command by placing your hand on your chest, then introduce the verbal command - watch me).  By using this technique, you aren't punishing him when other dogs are around, but rewarding him for the behaviour you prefer)  I think at the moment he's being over faced by other dogs just getting too close, which I appreciate is hard to control, but as the owner, you should be keeping an eye out for this situation arising and managing it.

Got a Halti and have been working with Dobie. Dobie is doing really well with the Halti though we have yet to encounter another dog.

Meet some dogs tonight. It was SOOOO funny. He sees the other dog, barks, goes foward as if the attack and the Halti gets him. He's so distracted by the Halti he misses the dog going by for the most part. He bucks and "fights" on the end of he leash. He is so funny. He does prefer that we run versus walk. I think part of the problem is we just don't get out enough w/ enough excercise. We don't have a fenced yard yet and so he is limited to when he goes out with us.

Have you taken the time to get him used to the halti?  The "fighting" at the end of the leash and the bucking isn't funny IMO, it's him telling you that he is extremely unhappy with this awful thing on his muzzle.

I'd take some time to get him happy with it - let him investigate it and give him treats and praise when he isn't bothered by it.  Hold it in front of his nose (not on it yet) and hold a treat through it for him, when he's okay with that, try to encourage him to reach through it for the treat.  Let hmi wear it for a few minutes at a time and praise him for ignoring it, then build it up.

If he's ticked off at the halti, he may well become worse on walks - heck, he may start to associate this thing pulling at his head (as he sees it) with the other dogs, as it happens when he moves towards them, and become more aggressive towards them in a kind of pre-emptive strike type thing.

How much exercise is he getting?  Dobes being the energetic little, *ahem*, sweethearts :) that they are (i have a 2yr old male), they need a lot - on leash, 2 or 3 hours, maybe less if he's getting a lot of stimualtion at home such as kongs, training etc; off leash, at least an hour if it's hard running (fetch etc, but best done in two walks so not to overwork him in one go).  When he's off leash, my boy gets two hours a day, half of it spent running (usually after rabbits!).  If you can give him enough mental and physical exercise, he might calm down a bit - be less wound up/frustrated.