Synonyms:
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"
Usage examples
This mercurial brigand, it would appear, has paid Turon another visit, but, with the exception of what may be considered the legalised robbery of the betting ring, has not levied contributions.
If one could divorce Edison from the idea of work, and could regard him separate and apart from his embodiment as an inventor and man of science, it might truly be asserted that his temperament is essentially mercurial.