I'll pipe in here too, as my wife has had very similar problems, also seemingly environmental-stress related, although usually it happens during her time of the month (right about now actually). Otherwise very similar to DJ's situation: completely out of control temper, a totally different person, a Jekyll-Hyde sort of change. She'll be normal and sweet one minute, then completely black the next, yelling sarcastically about how impossible her life is with us (me and our daughter, who is often present). Sometimes she threatens suicide, and sometimes she threatens to take a flight back to Germany with our daughter; in one case she actually packed two suitcases and was looking up Travelocity. In one case she obliquely threatened our daughter saying "You're ignoring me but I'm going to kill myself and you might wake up in the morning and find out that I'm not the only one gone" and in that case I called the police (the only time I did that). Since the police incident she has never threatened anybody, nor has there ever been any physical abuse.
In her case, she always gets over the incident fairly quickly, fully knows that what she did was wrong, and apologizes, and tries to be better the next time. I'm trying to get her some professional help but she categorically refuses. We take things one day at a time, and when she's fine (probably 90% of the time) she tells me "I'm so sorry for all the pain I cause you guys, I hope you can forgive me, you just need to remember that there are times when I'm dysfunctional." And then it happens again next week . . . .
I've talked to our daughter a lot about it, tried to explain, but I still feel it's totally wrong that she's allowing herself to behave like this in the presence of our 5-year-old. Our daughter seems to be taking it fairly well though, she just ignores her mom in these cases, usually goes out and rides her bike or something. Sometimes I take her for a walk (often with the wife yelling after us "Don't think you two are going to get away so easily!") and then when we come home she is usually embarrassed and apologizes.
I think the basic cause in her case was having a child, which for her has been an emotional stress that puts her over the top, and it changed her hormonally; she never had a regular cycle before, now she has the cycle from hell. She was the youngest child in her family, never had to take care of any kids, and this is an alien situation for her. I do most of the basic childcare stuff, she was too scared and panicky to do anything physical with our daughter when she was a baby; I bathed her, dressed her, changed her most of the time. I do all of the cooking, most of the cleaning. I have a great relationship with our daughter, while her relationship is off and on, with a lot of yelling on both sides. When it's good however, it's very good. She's a very wise, understanding, affectionate, thoughtful, loving person . . . . except when she's crazy
I don't have any solution, unfortunately. When there's a small child involved, everthing is infinitely more complicated . . . .