Dog unpredictably nipping on people, advice needed

[Olie]

Background:
Male, 2,5 y.o. shepherdish type, might even be pure bred dutch shepherd. Adopted at 1 y.o. Three-legged, leash and male dog reactive. Went through obedience training. He was pretty fearful of strange stuff, but now he's doing much better. He was never crazy about new people, especialy men. I sometimes think he was abused.
He was never aggressive to people, except on 1-2 occasions when he was in a pack and other dogs started barking on people, he then went for their hands, but didn't bite, more like he pushed the hand with his nose.
Problem:
A couple of times he attacked/nipped on different people in different situations:
1) about a year ago he jumped/rammed a man on a sidewalk, while he was passing me by. It came out of nowhere, no barking, growling, nothing. He didn't bite him but it did look like an attack. At that time I explained to myself he was frustrated because minutes before he saw a dog and later a cat and he was arroused by that and needed to vent.
2 & 3) he did exactly the same thing as 1) twice on different days to a neighbour boy. Poor boy is now terrified. I thik he might remind him of someone from before adoption.
4) & 5) First nip! May 1st: I took him to our country house for the first time. Spent a night there alone with him and the next day I had some friends over. In 2 min appart he nipped 2 of my friends in exactly the same way: he was laying peacfully in the house while they were walking up and down the house. He sneaked from behind and nipped their buttocks. I was flabbergasted. He spent the whole day leashed from then on. (btw he tried to do the same thing to some workers we had in our house moths before, but my dad saw it and yelled NO and prevented it. We thought that was territorial + he felt something strange about that guy. Worker also said he tried to bite his hand while he was working. Strangely he was fine with other workers)
6) A few weeks ago: He nipped my dad on exactly the same way, same house, same everything, just like he did my friends. What's strange is that he LOVES my dad and vice versa, we live in the same house, dad takes care of him when I'm not home etc.
7) 2 days ago: on a walk a young man approaches and asks if he'd bite. I said "no, but he doesn't really like people". Guy goes on and pets him on the had, while asking how he lost his leg etc. The dog moved closer to him to sniff him (maybe the guy thought that ment the dog liked him). He then tapped him on the rib cage, and pulled his hand away, as leaving, and as he did that, the dog went after his hand snapping the air a couple of times. He even made a couple of steps after the guy, trying to catch and bite him. He was leashed so couldn't get to him. (though, i think if he really wanted to bite, he would, because he is usually much faster and determined, i.e. catching the ball, (unfortunately) attacking dogs etc. I think he wanted to scare him a bit). What I think irritated him is that the guy was very confident around him, like they are old friends, and especialy that he touched his rib cage. In fact, couple of months ago my uncle said the dog growled at him hard when petting him on the ribs. We though he probaly doesn't like being touched there, but lets us do it beacuse we are his people).
He also growled and faked a bite when my grandma wanted to touch his bed, so she doesn't do that any more.
What do you guys think? What should I do? I don't think I need to muzzle him since it's not very often or very serious, but I can see something is going on. People say I'm not enough authoritorative in general, bit I can't see how that would improve this behavior. He obviusl has some impulses to bite, but he's also inhibiting them by not going all the way. All of those "attacks" happend so unpredictably that I was stunned and confused and I didn't really scold him (except the second time with the boy. Funny thing he looked very pleased with himself.).
What should I do?
P.S. There was one sitution where I was sooo affraid he would react, but he didn't. I had to take him to a certain celebration. It was in unfarmilliar place, a lot of people, cameras, lights, people clapping, people touching him mindlessly, people pushing their faces in his face, crowd, etc. He was pretty upset, but he did very nice.

[Stacy_R]

2 things:
1. Find a behaviorist in your area who can see him in action. It's hard for anyone to say what to do without seeing his body language in these situations e.g. is it a space issue?, is he sensing something in you?, is he in pain? has he progressed from reactivity to aggression issues or do you just need to redirect him?, the possibilities are endless.
2. Take him to the vet to see if there is any medical reason for his rib sensitivity. If not, you might just need to do some conditioning with that area of the body. Again, a behaviorist can help you here.

Good luck!