General:
* Erratic
* Unstable
* Volatile
* Capricious
* Whimsical
* Random
* Uncertain
* Changeable
* Inconstant
* Fluctuating
* Mercurial
More specific:
* Unforeseeable (emphasizes lack of ability to anticipate)
* Unreliable (emphasizes lack of dependability)
* Unpredictable (can also refer to a person's behavior)
* Inconsistent (emphasizes lack of regularity)
* Unruly (emphasizes lack of control)
* Tumultuous (emphasizes chaotic nature)
The best synonym will depend on the specific situation and the intended meaning.