erratic, fickle, mercurial, quicksilver(a)
Meaning: liable to sudden unpredictable change; "erratic behavior"; "fickle weather"; "mercurial twists of temperament"; "a quicksilver character, cool and willful at one moment, utterly fragile the next"
Meaning: marked by erratic changeableness in affections or attachments; "fickle friends"; "a flirt's volatile affections"
Fortune is very fickle."
Many girls might have been taken in, for never were such attentions; but I knew the fickle sex too well.
I make but a poor figure at composition, my head is much too fickle, my thoughts are running after birds' eggs, play and trifles till I get vexed with myself.
But nevertheless I at no time entertained any ill-will towards you, only you were much favored by Prince Genji, as I heard, which made me abstain from visiting you often; but fortune is fickle, for those in a humble position often enjoy comfort, and those that are higher in station are not quite so well circumstanced.