If you want my opinion then here it is.
I think she is probably borderline with narcissistic tendencies.
(Based particularly on, she does act nice to you sometimes, seems like this is mainly just when she is the right mood, or probably so.)
She could have a mood disorder, or borderline personality disorder, or narcissistic personality disorder, or some combination of the above.
Bottom line though. Whatever is wrong with her.
For your own peace of mind, you probably shouldn’t stay with her any more.
Get out, move on, and find somebody else.
Preferably as soon as reasonably possible.
Good luck.