General antonyms:
* Inconsistent
* Variable
* Fluctuating
* Erratic
* Unpredictable
* Irregular
* Unreliable
More specific antonyms depending on context:
* Sporadic (if referring to frequency)
* Inconstant (if referring to loyalty)
* Intermittent (if referring to something that stops and starts)
* Changeable (if referring to something that changes easily)
* Unstable (if referring to something that lacks firmness)
The best antonym will depend on the specific context in which you are using the word "consistent."