General Synonyms:
* Sporadic
* Occasional
* Periodic
* Episodic
* Recurrent
* Irregular
* On and off
* Fits and starts
* Discontinuous
More Specific Synonyms:
* Fluctuating: Used for things that change in intensity or frequency.
* Interrupted: Used for things that are stopped and started again.
* Alternating: Used for things that switch back and forth between two states.
The best synonym for "intermittent" will depend on the specific situation and the nuance you want to convey.