The phrase "on again, off again" is used to describe an intermittent or unstable situation, often referring to a relationship that involves frequent breakups and makeups. Some synonyms for this phrase could include "sporadic," "inconsistent," "wavering," "unreliable," "intermittent," or "irregular." All of these words convey a sense of something happening irregularly or unpredictably. Other phrases that could be used include "hot and cold," "stop-and-start," or "hit-and-miss." Whatever the phrasing, it suggests a lack of consistency or predictability, which can be frustrating or exciting depending on the context.