First post so be gentle with me ladies 
This may end up being long so 
if you make it to the end!
Have been part of a small, and extremely close group of friends for over 10 years now, only four of us and we have been best friends since school. I will call these friends A, B and C just to make things easier. I must stress that the friendship with this group means a great deal to me, so would love some advice on how to deal with this.
Particularly since university, we have found some of A's behaviour to be emotionally draining and hurtful. She has always been 'feisty', but in recent years she has become aggressive, judgmental and hypocritical. Every time we meet up for lunch or something, she spends the entire time talking about herself, and if any of us tries to direct the conversation elsewhere she sits looking bored and quickly finds a way to bring it back to herself. Since splitting up with her ex-P, she has embarked on a series of 'friends with benefits' type relationships and one night stands (NB, none of us are 'judgy' about this - if she's happy with it, we're happy iyswim) but she has become attached to the point of obsession with one of these men, and every time we see her she complains about him and asks what we think she should do. Any advice we give is sullenly met with, "yeah but I'm going to do what I want to do." We're always happy to give advice or even just to listen, but when the 3 of us all have very real problems in our lives, it feels a bit insulting to have our friend wail at us about how unfair her life is, when it seems that all she does is get drunk and sleep with men.
Throughout my entire relationship with my now ex-P, A would constantly talk about my ex-P in a derogatory way, and every time I saw her she would try to convince me to leave him "because he's ugly and if you stay with him your kids will be ugly too". She would phone me drunk from parties saying "I don't want you to be with him." I told her this made me uncomfortable but she didn't stop. I even wondered whether he had tried it on with her and this was her way of trying to tell me, but she said this didn't happen. I have asked her why she had it in for him since we split up (split nothing to do with her!) and she said "I just didn't think he was good-looking or particularly nice." 
And knowing her as I do, I know that she would just tell me if he had been inappropriate with her.
Keeping this in mind, another of these women, B, is in a verbally and emotionally abusive relationship and is very emotionally vulnerable. At one of our recent meets, B's DP was calling and texting her with abuse and telling her that she wasn't allowed to come to the next thing we were planning (he does this pretty much every time she goes out or comes over to one of our houses). Me and C gently tried to talk to her about it and discuss his behaviour whereas A shouted us down and said, "You two can't tell her to split up with him, you can't help who you love. B, ignore these two, I'm on your side" etc etc. We hadn't been telling her to split up with him or anything of the sort and we were furious that she was pushing a vulnerable woman back towards her abusive DP by implying that her friends 'aren't on her side'. That's what her DP wants her to think as well!
She is always moody if any of us have to cancel meeting with her because of family emergencies, illness etc even though I have been v apologetic on the few occasions I have cancelled over the years. However, often when we are about to meet for lunch, she'll text one of us saying she's too hungover or something similar.
Probably the worst thing is her tendency to 'go cold' on any one of us, seemingly just because she feels like it. She will ignore calls and texts, blank the victim at any group meets, etc. This can go on for as long as she wants it to - eg. a few years ago, she went cold on B for almost 2 months until B couldn't take the silent treatment anymore and burst into tears and asked what she'd done - it turned out it was because B had taken a long time getting her coat from the cloakroom at a club and A was bored waiting. She also went cold on me for several weeks because I had said her ex-P looked hilarious in some trousers he was wearing (NB we were at a fancy dress party, this wasn't intended as an insult!)
Last year neither B nor me could make her birthday celebration (we did do a special dinner with her but this was some clubbing thing, mostly with her work friends) and she told us it was absolutely fine, but spent the whole night bitching to C that she could no longer trust us because of this. However, the night was hell for C anyway as A showed up late to her own party, spent a lot of it throwing up and virtually ignored C. To add insult to injury, A then spent C's birthday out clubbing with her work friends instead of seeing C. She would go mental if any of us did the same thing.
Sorry this is so long, and you deserve 
for getting to the end. C and I are pretty much at the end of our tether so I just wanted to include a few examples of her behaviour and ask whether these things are petty or if its reasonable to confront her and tell her the 'going cold' and aggression has to stop, and to face the consequences of confronting her, whatever they may be?