My mother sounds incredibly similar. Her moods and reactions were so unbelievably unpredictable when I was growing up-- if I made a mistake I never knew if the kind, loving, forgiving mother would respond or the angry hostile one who would hit me.

As an adult, I work in the field of mental health as a psych nurse (go figure.) I'm not a psychiatrist, but if I were going to diagnose her, I'd say she's got a pretty serious borderline personality coupled with depression. BPD's are pretty classic for those "love or hate" relationships, unpredictable emotional reactions, etc. etc. etc...