Unpredictable:
* capricious
* fickle
* whimsical
* volatile
* unsteady
* inconsistent
* unpredictable
* changeable
Irregular:
* sporadic
* intermittent
* irregular
* disjointed
* unstructured
Unstable:
* unstable
* unbalanced
* inconsistent
* flaky
* unreliable
Other possibilities:
* aberrant
* anomalous
* bizarre
* unorthodox
The best synonym for "erratic" will depend on the specific situation and the desired tone.