Helping a dog who shows random aggression, probably through anxiety or fear-?

Has anyone who rescues, trains dogs - or a caring pet owner- ever had to put down a dog for aggression? I've never come across a dog that couldn't be rehabilitated in all my years of rescue; however, there is now a dog so unpredictable (sweet as can be, then suddenly turns, growls, shows teeth for no reason anyone can figure out) that 3 people have given up trying to change him. He is perfectly healthy, fully vetted down to a very expensive dental, plus he's neutered. He came from Animal Services, and he's been passed around 3 times in a month, so I put it down to anxiety - but this is more than that, sheer instability I suspect, goes from loving to vicious in a split second, even with people he seems to love dearly. If anyone has come across a similar situation with a dog, I'd love to have your advice, thoughts, experience. Anti-anxiety drug busporin has been suggested, as has just ignoring him when he gets like that, but it's hard to ignore a dog who is acting like a direwolf. I'm going to take him on, but any practical advice would be much appreciated.
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Unpredictable is VERY hard to work with. My chi is unpredictable, and I ended up just keeping her, because I can read her and handle her well enough to not get bitten. While she is much better than she was when we first started, I will probably never completely trust her with people she does not know. My current foster is also very unpredictable - she warns most people she does not know away, but will throw herself into the arms of the occasional stranger. She loves most kids, but I do not trust her with them. She is fine with me for weeks at a time, and then has a meltdown over something simple, like getting her nails done or getting a bath. I swear this dog has moods that change throughout the day. I am having a hard time picturing the person who will be okay with her, but I am not planning on keeping another one, either. :o) You dont mention if the dog is a large dog or small - that would be a factor for me being willing to work with a dog. A small unstable dog can only do so much damage. If you want to email me, I will be happy to share what has worked for me, and what has not, but its possible this dog may be the one you cant reach.
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