impulsive and unpredictable
capricious - erratic
The most frequent use of the word ‘capricious’ is to describe a person whose opinion, mood, or behaviour can change suddenly and without warning. Often times, such people will decide for or against doing something on a whim rather than after careful thought. The term can also be used to refer to weather or an environment that is similarly unpredictable.
I would never want to go into business with John. He’s far too capricious.
That part of the country has quite a capricious climate, so its hard to predict what the weather will do.
The capriciousness of the markets makes investment a risky proposition.