I'll try to be reasonably brief, because it's a bit of a saga.... I have a 19-month-pld lurcher who I have had since he was 7 weeks old. He's our first dog, not counting a couple I had as a child. As far as I could tell the litter were well cared for, and from the start he was raised using positive methods. He was a very high-energy pup, mouthy despite all our efforts to teach bite inhibition, and clingy.
As early as around 12 weeks he started showing food and possession guarding behaviour. He's always been fine with toys, but as well as food he would guard anything he stole, such as socks. This is despite the fact that we never removed food from him, though we practised exchanges and taught him 'leave' in a positive fashion. We followed his trainer's advice, but to be honest the behaviour got to be so bad so quickly we didn't have time to work out an appropriate strategy to control it. 'Trading up' wouldn't work: at his worst, at around 6 months old, he once nicked a plastic measuring spoon and I offered him a lamb steak in exchange, but he flew at me and bit me on the shoulder. He also attacked me once when he had a pair of my son's underpants and I just looked at him wondering what to do.
We worked with a positive bevhaviourist, and made some progress, but in February he had a disagreement with my husband over a bread roll in a carrier bag - Jasper was on lead but managed to lunge for the bag in the gutter, hubby pulled him away and Jasper dropped the bag, Jasper then had a tantrum and bit my husband in the hand badly enough to need to go to A&E (though no stitches were needed). The behaviourist then said that Jasper was 'on borrowed time'
We have a new behaviourist and have made further progress - if he has a kong I can call him away with a call of 'sausage!' or hand him a piece of food while he has a kong, and we've even started work with bones, though so far all I do is walk past him and throw treats as I approach. He's stopped guarding 'stolen objects'. Almost all of the advances have been done by allowing him to feel relaxed when he has something and calling him away/distracting him if necessary - as he is so suspicious I have largely avoided working on 'leave' or exchanges for high-valued items. I also feel that 'pestering' him when he has a kong by asking him to exchange it, even for something better, will instantly induce stress - at least for now.
He is also very highly strung in certain situations - he freaked out when we tried him at lurcher racing and tried to attack the man who releases the dogs (leading to a lecture on how we need to pin him down and make him submit
). We've tried showing him, but he'll turn into a kangaroo in the show ring. He's nervous of children and has snapped at them a couple of times. He sometimes also develops a fear about going to our local woods - I haven't quite sussed that one out, though I have one or two theories... He completed beginner's and intermediate training classes, but was always hyper in class and would turn into a kangaroo. Last summer we had a huge problem with him jumping and biting our arms on walks, but now he only does this when he's overexcited/stressed and can't control himself - often he will run towards me with that look in his eye, but then he'll pull up at the last moment - particularly if I say 'Ah-ah!' and hold up a finger - and then I'll reward him for not jumping up. In general, despite all this, he is obedient and well-mannered round the house. Being a lurcher he's an incorrigible thief and I can't leave anything out, but he will leave the kitchen when I ask him to. If I drop a treat on the floor in front of him and he thinks I did it intentionally, he will wait till he's told he can have it, but if he thinks it was an accident he'll grab it at lightning speed
He's always muzzled when we go out because apart from his fear of children, he's cheeky enough that he would nick food out of a passer-by's shopping bag (he once got a pork pie this way, and he has also nicked someone's car keys and a woolly hat!), but he would bite them if they tried to take it back. He also scavenges and if he found a bone while off lead and a stranger approached him he could attack. And I don't even want to think about picnics.....My real worry at present, is that he is easily stressed and easily frustrated, and will use his teeth when this happens, even on me. If he decides he doesn't want to go on a particular walk and I pull him along, he may well jump up and attack me, and if he wasn't muzzled he would do serious harm - I really don't think he's playing
This last week has been particularly bad as he hurt his wrist and has been on lead walks all week - he's had several of these tantrums. He's also growled a couple of times, both on having his muzzle put on and taken off, though he's usually absolutely fine with it. It's easy to say that he'll be fine once he can run properly again, but there will always be occasional stressors in his life. So, having got to this stage, with all the work we've done, it doesn't seem right that we have a dog who seems loving, trusting, well mannered and obedient almost all the time, can still attack his owners in a way that would have probably seen him put down by now if it wasn't for muzzles 
I suppose before his injury we would see, maybe, one show of aggression every couple of weeks, so this week has made his problems seem particularly bad.A couple of other bits of information - he's about 30" to the shoulder and very long, and 31kg. I'm 5'0" and about 8 stone, so when he does jump up on me, I know about it! I normally stagger to the nearest tree or fence and wrap his lead round it, and ignore him till he calms down. He was neutered at 10 months - rather early (depending on who you speak to), but I thought it essential because of his aggression. He generally gets on really well with other dogs - if he sees one he's not sure about he'll hide behind a tree till it's gone by
Any suggestions for further strategies would be very welcome, or even just 'He sounds like my dog but he's getting better with age' or something... I'm a regular on a lurcher forum but feel a bit alone as lurcher are normally so gentle! According to the behaviourists, I haven't done anything to cause this behaviour and it must be either genetic or early experiences - I've since met his mother and sister and they both growled at him
Don't think they've ever shown aggression towards their owner though. They live in a multi-dog household and I often wonder if Jasper would have been better in that sort of environment, where he could have learnt from other dogs. They both hunt rabbits, too, but I can't see anyone getting a rabbit off Jasper if he managed to catch one...His normal routine (when he's not injured) is that he has a walk of around 1.5 hours in the morning, and similar in the afternoon. I work at home so am around most of the time - as he's clingy I make a point of going out most days - I can leave him for 2 hours, but it's taken a long time getting there! We'll usually have some sessions of play and training during the day. He's raw fed, as no kibbles suited him (sloppy poos!), and has had full bloods, thyroid, etc done at the vet's. I tried Zylkene - he seemed too quiet and quite depressed, though I'm not 100% sure that was the Zylkene (it was over the winter and the vet thinks it may have been seasonal affective disorder!), and it didn't reduce the aggression.
OK, so it wasn't so brief..... Sorry! And thanks in advance for any suggestions.
and here is the good news - he will grow out of a lot of the behaviour that is driving you nuts just now. But not all of it. I could nuke the people who are crossing Irish Terrier into lurchers because they are creating a combative belligerent mindset in what is essentially a gentle and sensitive type of dog. The Bedlington is all terrier too but not so confrontational, and works better in the lurcher mix. All this is hindsight and explanation: you have to deal with the dog as he is now.