Ah, thanks. This might explain it.

Yep, cycling between baseline and "the black hole" is pretty much how it goes, at least based on external appearances. I've also witnessed mood shifts (not really swings) that can come on suddenly, usually provoked, and last for a few days or a week...it kind of scared me when I was younger, I didn't know what I was going to come home to when I came back from school. Pleasant chit-chat? Angry yelling? Hostile silence? Whichever, I had a 50/50 chance of getting an explanation for it.