I do not, by any means, condone Buffy's actions in Sanctuary. However, I do understand her reasoning for such behavior.
Regarding the scene in which Angel consoles Faith, yes, Buffy over-reacted. If one paid attention to what occurred prior to Buffy's arrival, you will remember seeing Angel stepping out of the shower to check on Faith just before she became engrossed by the news bulletin concerning her location, as well as being attacked by Wolfram & Hart's demon-assassin. Less-than a minute after Angel's shower, Angel heard a crash while he was getting dressed. Without hesitation, Angel rushed to Faith's aid as he put on his last article of clothing. After the demon was taken care of, Angel saw Faith's distraught look as she gazed at the demon's blood upon her hands. Presumably, the demon blood reminded Faith of her past, which thwarted her into confronting her problem, and thus Angel comforted Faith. Buffy then enters the room, only to see what appears to be her first love, ex-boyfriend and lover holding an unknown (for a few seconds) woman.
Buffy: ''How... What are you doing?''
Angel: ''She - we were attacked.''
Buffy: ''We? - You and...''
Angel: ''Faith.''
Buffy: ''You and Faith.''
Angel: ''Oh, it's not what you think.''
Buffy: ''You actually think that I can form a thought right now? - Giles heard that - that she tried to kill you.''
Angel: ''That's true.''
Buffy: ''So you decided to punish her with a severe cuddling.''
Judging from the camera's angle, and Faith's position, it was difficult for Buffy to recognize the woman on Angel's bed, and this is why Buffy pauses (as her eyes wander to the bed), only to have Angel reveal Faith's identity.
Buffy wasn't just jealous, and she wasn't just jealous because she saw Faith and Angel together in the same room. Buffy was jealous before she knew who was with Angel, but finding out that it was in fact Faith didn't help the matter at all, and if you include the apparent scenario, her reaction is all the more understandable. Buffy was upset (with righteous reasoning, yet again) with Faith when she slept with Riley in Who Are You?, and Buffy only cared for Riley. But, this is Angel, and the difference lies in that
she loves him.
So, yeah.... the mere thought of the love of my life sleeping with my enemy would tend to make me a bit cranky, too.
As for Buffy visiting L.A. to seek revenge, I don't believe that. I believe Buffy wants revenge, sure, but that wasn't her main priority contrary to popular belief; her main priority was to help Angel, if not to ensure his safety.
Lastly, in the police station, both Buffy and Angel were at fault.
Angel was at fault for saying, ''Buffy, this wasn't about you! This was about saving somebody's soul. That's what I do here, and you're not a part of it. - That was your idea, remember? We stay away from each other.'' Bringing up such an issue is obviously going to lead into a conversation about their past romantic relationship, which is touchy for the both of them. Knowing Buffy, Angel couldn't have possibly thought that Buffy would let such a statement just roll off of her shoulders.
Because Buffy was hurting, she mentioned her life with Riley in retaliation to what Angel already said. I believe she told Angel about Riley because she felt it was time to do so, and that she might as well get it out of the way as opposed to waiting, because that could only complicate things further. Additional retaliation ensues when Buffy takes a stab at Angel by saying that she ''knows" and "trusts'' Riley (Buffy feels betrayed by Angel because he didn't mention to her that he was protecting Faith; one assumes Buffy is mentioning ''knowing'' Riley because she feels he would never go as far as to betray her), which ends in Angel attempting to hurt Buffy by saying she "doesn't know" him any longer.